Abody rear springs sagging already

I spent some decent dollars on rear stock springs from Eaton Spring in Detroit, maybe 6 years ago. I've put about 1,500 miles on my 71 Demon340 in those 6 years and I would like the ride to be an inch (or so)higher. Is it possible to have someone add one more spring to the existing and get a little lift? What are my options? Thanks

I added springs (one on each side) to a B-body I had, it worked great & it is easy to do.

I did have a donor set here from another car though.

As rule of thumb, adding one full leaf adds an inch of installed height. My local spring shop did this service on my sagging MP XHD springs for $100. You will also get a higher spring rate... which you may or may not want/like. In my situation I needed the higher rate.

There's different ways to change the heights.
Stiffening the rear springs is not a good way. Doing so increases the tendency to oversteer. Oversteer will show up in the worst situations such as, downhills, at high speeds, and/or on slick surfaces.

Increasing the rear arch can be done cold or hot, but it will not likely hold up as long when done cold.
Increasing the rear arch can improve launch on the drag strip if properly combined with sticky tires. It is also used sometimes by circle track cars in combination with other changes to improve coming off the corners.
On a production car, arch can have negative handling effects.
Flat is basicly correct. This is how Chrysler designed 'em. Chrysler explains it in the this booklet about handling.

Trying to find the original rear ride height is difficult.
The only rear height info I've seen from the factory is at Hamtramck Historical.
Bumper Heights
Not really sure the intended purpose of this document as these heights were never in service manuals, but its better than nothing when you're not sure. Note that on page two that these are for fully loaded (5 passengers, full fuel) but does not say what options. Also the bumpers are somewhat adjustable and the body has some uh slop.

That said,
if you carry significant extra weight in the trunk, stiffer rear springs will help. The additional stiffness will reduce the deflection from the extra load and the extra weight will mostly offset the oversteer tendency.
if you drag race, some additional arch, a thicker main spring, extra front clamps, and a strong second spring under the front eye will all help launch and reduce spring wrap. All but the arch will also help the spring survive longer.

Mattax, good info, thanks. My leafs are almost flat. I bought a 71 Demon340 in '71 and my ride height today is what I had then. I have gone to 15x8" rims out back and think I want it an inch higher.

On my Cuda, I used Hellwig helper springs to raise the rear. Been on there a couple of years now and they have done fine. Rides about the same and I can adjust them to most any height I want and don't have to worry about sagging anymore.

When I installed them (they go on the leafs behind the axle) I installed them on top of the leafs so that they don't show.

As Ahy and Mattox already said, extra spring will change the spring rate.

Think of it as increasing coil thickness on a coil spring. Not the solution if height is what you want to change.

Rearch is the best solution.

The best approach will depend on nailing down why you want it a higher. That only you can answer.
If you take this to the track (drag strip) then changing the angle between the axle and the spring eye can reduce squat or even get lift - which is what is needed for best weight transfer onto the rear tires. It may also need the pinion snubber run up to the floor. Running on the snubber is not great for street or cornering but its a tool to use when launching on leafs.

If you're just doing it for a certain look, that's fine as long as you keep in mind that you must be extra careful in situations mentioned in my post above. Don't ask me how I first learned this.

If you do increase the rear spring rates without additional rear load, then consider significantly increasing the front roll rate.

If I remember the old DC bulletins correctly, that's what you want. If you have a positive arch then you get body roll in corners.

(Had John R spring add one leaf per side to my OEM spring packs...

Doing fine 20 years and 20,000 miles later...

I'm a fan of a bit stiffer springs, so i would add a leaf, but i would also put some heavy torsion bars up front or a decent size sway bar to balance it out.