My '72 Dart project had minitubs and a pair of unknown springs when I bought the shell. Today I put the car on the ground and drove it a short distance. But it looks way low in the rear (raked?) and not even symmetrically. The shocks only have about 1.5 and 2.5" of rod exposed.

How much arch should there be in a stock spring? Or is this pair just not heavy enough for the car even though it no longer has a back seat?

Those look like a H.D. spring set(not stock OEM light duty 5 leaf springs) with 6 leafs on each side, is there a part number stamped into the bottom leaf facing down?
What does the rear shackle angle look like at rest, straight down angled to the front of the leaf spring or angled back towards the rear bumper?

Springs could be tired. I count 6 leaves in each side. The left side is almost flat. One thing you could check is with the car on jack stands supporting the frame, on a level floor, measure the distance between the front spring eye bolt holes to the ground. Should be the same. And if there are multiple holes in the box, insure both sides mounted in the same relative hole. Insure the rear spring hangers through bolt hole positions are the same as well. If everything measures up then it's time for some springs. Or throw another leaf in each side. You might also check for part numbers on the springs and post them here. MP SS springs have a left and right side orientation. Usually stiffer on the right spring than the left. 6 banger and 318/340 stock springs usually have the same pn on both sides, in my experience.

I will check tomorrow for part numbers. The rear shackles are angled backwards about 2" (spring eye behind the frame end of the shackle). The spring eye looks close to the frame rail. Is that how they're supposed to look?

You want the bottom of the rear shackles to be rearward from plumb (vertical) otherwise if it is in front, the spring will raise the car as it compresses in that arc over center.

ok, then the shackles are angled correctly. I suspect these old springs are just tired.

The battery is in the right rear of the trunk, and the fuel cell in the center. Stock heavy bumper. So back to my original question, shouldn't there be an arch visible in the spring? As pointed out, they look nearly flat just sitting there.

Something else occurs to me - if I don't have the ride height on the torsion bars set symmetrically, will that force the rear to sit differently too? Haven't checked for that yet.

The more heavy duty the springs are, the less arch they tend to have. If there is a negative arch to the rear spring, you may have some broken leafs in the spring pack. The leafs tend to break at the center bolt, which would be mid way between the the sides of the U bolts. One or two broken leafs in a spring will cause a pretty bad sag. If you bounce the back end of the car, does it return to the same level?

The reduced shock travel probably means someone put a set of shocks off a B body on your A body car, or something else is not right. I've seen broken springs with more shock travel the 1"-1 1/2". Generally, there should be a bit more then 4" of shock travel before the shock would bottom out.

Incorrect torsion bar adjustment will have an effect on the rear height, side to side. If the left torsion bar is significantly lower then the right torsion bar, the left rear will be lower then the right rear. Gene

I'll look them over closely for broken leaves. No negative arch, just awfully flat looking... yes, bouncing does return.

I bought the rear shocks a while back, through the classified ads here on Moparts (Calvert Rancho 9-way adjustables). They were supposed to be A-body, and they did bolt up and have enough extension to let the rear hang with the frame on jack stands. Didn't occur to me to check compression! Looks like I should check their part numbers, too.

Mopar springs, when installed and the car is on the ground, may appear to be nearly flat. In most cases, the installed arch is no more than 2". This is by design for improved handling purposes. Exceptions to this may be station wagons, trucks, and of course, drag race competition springs. Mopar oval track competition springs may actually have reverse arch to them.

From what I can see in your pictures, they appear a bit flatter than original, but not radically so. It could even be camera angle. Your overall ride height in relation to the wheel appears okay. The extra leafs mean they also have a higher rate so even though they may sit a bit lower, they have more resistance against bottoming. Judging from the length of your u-bolts, and the leaf count, these are not original.

If you have competition shocks, they are designed for spring with more arch. You may not be using the best range of their motion, but may still be adequate.

Unless you are bottoming out or bouncing around, there doesn't appear to be any major issues going on here. If either of these are the concern, they can be addressed with a change in either the springs(bottoming) or the shocks(bouncing). If you are simply unhappy with the ride height, then that can be changed with a spring with more arch(camber).

You may be able to flip the front spring mounts to gain some hieght.

Thanks for the suggestions everyone. The previous owner did (or had done) mini-tubs and relocated the front mounts into the frame rails. I need to take a close look and see if anything can be moved.

The bigger problem is that there's 1.5" difference in the ride height side-to-side... I suspect a cracked leaf, or just plain worn out.

If your front mount is i the frame rail, the flip trick above is not an option. Depending on their accuracy in doing the spring relocation, there may or may not be issues with the front hanger location. Some people put two mounting holes in the relocation kit, some only put one. I'd certainly recommend checking these mounting locations for the front spring eye and ensuring they are equal and/or try changing their mounting hole if you have a choice of two.

If your leaf mounts are indeed symetrical, you can offset the rear height difference by adjusting the front torsion bar bolts. The corner opposite of you high leaf can raise up a turn or two. This will tend to bring down the high rear corner and level out the view from the back.

OK, but what will that do to the front heights? Offset them too, right?

I am not so sure the amount of arch is as crucial is as how the springs move when in use. Meaning, maybe stating the obvious, any compression movement that that moves the rear axle forward on the outside wheel in a turn, reduces oversteer characteristics, and any movement that moves axle rearward under compression on the outside wheel of a turn, promotes oversteer, on leaf spring cars,that move the axle thru a small arc under different loads. The inside wheel is the opposite, but plays less effect since it carries less load/grip when cornering. This should be easy to visualize.

This is why production springs have very little arch, for toe control.

Drag race springs (without traction aids) generally want stiffer front segments, a higher front eye, and bias for launch control.

Today I measured the front ride heights... and found to my surprise that the left was much lower than the right! At least an inch. I knew it would have some effect, but I'm surprised at how much difference it made to the rear heights

To fix it I had to disassemble the adjusting bolt & block since the threads were galled and would barely turn, but with the help of my Lang (USA made) rethreading set, it's adjustable again. The damaged bolt threads are past the area of adjustment that I need anyway... something else I probably knew about and forgot to fix several years ago.

Now, with the fronts equal, the rear is now just about equal too. But they are both too low (the bead edge of the rim lines up with the rear quarter lip. I want it a couple of inches higher anyway, especially since the front can't be down on the deck with my 8 qt pan, Lakewood and underchassis headers/dual 3" with X-pipe exhaust.
Looks like it's time to order a pair of springs!

Edit: there is less than 1.5" compression travel on the Calvert/Rancho shocks and they bottom out with a good push on the trunk. Definitely designed for a higher ride height...

Now you see the impact of front ride height on rear height. Diagonal weight tranfer acts much like a see-saw. If one corner is too high, adjusting the opposing corner can change it.

With a min-tub and inboard springs, its a distinct possibility a previous owner had competition springs on it, which have considerably more arch than stock and would be consistent with the longer travel shocks.

If you want a taller overall ride height, there are only two ways to get there, change the mounting location of the springs, or change the arch of the springs. Changing the arch in the springs can be done by have the pack dis-assembled and re-arched or swapping in a higher arch set. Higher arch sets can be custom made with whatever arch and rate desired, from a place like Springs and Things, or you can go the simple bolt in Super Stock route.

If it were me, I would upgrade to Caltracs and his springs. You have not posted a side view of the car so it is hard to get a good idea of ride height.

I'm thinking of buying a set of Caltrac bars (especially since I already have the shocks) They work with stock springs too, don't they?

You have to fix your springs already so why not get the right ones to start? The bars will work with stock springs but not as well as the monos. I know because I have done it. Get the whole package from Calvert.