This is how i'm going to do mine. Honestly i cant see the point in bolt-on connectors. And welding on bolt-on connectors is wasting a lot ov metal (needed to make the bolts work, and not needed when you just weld), and adding unnecessary weight. I also think its funny that people so obsessed with originality wont weld up a little spot (you CAN craft e'm so you dont have to weld the whole floor... just the ends), but will drill all kinds ov holes. Anything can be reversed... even minor welding. Not that you'll want to... you'll forget all about that originality once you drive that sold new chassis...

This is how I did mine too in my 71 B.

It is just as much, or less work than the US Car Tool ones. To me the US Car Tool connectors are only as strong as their skinniest section, which will be where the floor sits the lowest. I am much happier that I kept solid 2x3 tubing from end to end, instead of connectors that thickness depends on the height of the floor pan.

I guess if this was a 4 door C body, that was specifically for family use or loadin up friends, I would consider the Car Tool ones so the floor remained even in the back for passenger comfort (no tube sticking out of the floor where your foot usually goes). But since I have a 2 door, and you'd have to be pretty brave to sit in my stock back seat that has no seat belts with me at the wheel, I have zero regrets with straight tubing.

The only thing I suggest no matter how you go is make sure the frame is straight. We did them on a 4 post drive on lift and used jacks to bring the frame up to fairly equal height off the ground, I think within 1/8 inch, but Im still paranoid that the frame wasnt perfectly straight.. and once you tie the frame its gonna stay that way. a frame straitening machine could probably muscle a tied frame car, but it can get expensive.

To me, these uni-body cars are like 3 boxes taped together in a row. Once you stiffen the center box with subframe connectors and torque boxes, which is a no-brainer except for some high-dollar, trailer-queen stock show cars and historic examples.

The weld-in subframe connectors and torque boxes make the biggest difference in how the car feels on the road, even with a Slant six. Its nothing short of dramatic. Bolt-ins are best left off the car.

The rear box section of the car is attached to the cabin of the car much better due to the roof, quarter panels and other such internal structure. It doesn't transmit much of any sensation of torque deflection to me. In very high horsepower cars with twin turbos or some other huge amount of power, it needs to be reinforced.

The front clip is another story. The engine weight is there, the attachment points to the cabin are much less significant and there are a lot of things working to twist and deflect the structure under power and on the road.

Reinforcing the attaching points from the cabin to the front section or "box" yields tremendous benefits in terms of enhancing the overall solid feel of the car. The US Cartool Chassis Stiffening Kit is a great addition to this area. Welding the shock towers solidly to the inner fenders and welding in the under fender bracing and front frame rail tie-in bar makes a huge improvement in increasing the bond and reduces cowl shake dramatically.

Finally, adding a triangulated Monte Carlo bar to the engine compartment finishes of the package very well. One must be creative to accomplish this without hacking up the car. I was able to make one that can be unbolted leaving little to no evidence that it was ever there. There is a noticeable difference in chassis feel on the road when it is out.

Do you have a picture of your Monte Carlo bar?

Monte Carlo bar - triangular structure that connects the firewall with each shock tower, and the shock towers to each other. First seen in early road-racing Mustangs, if I'm not mistaken.

It is possible to put in a length of smaller square tubing that goes beneath the floor low points, connecting the larger rectangular tubing ends. As we are most concerned with torsion, it would also be possible to run the rectangular tubing with tall axis horizontal where it passe4s beneath the floorpan.

You guys are once more applying "common sense" or intuition to an engineering problem. If you really want to go farther, here's some help from a textbook. You could also just use square or round tubing. http://www.uap-bd.edu/ce/anam/Anam_files/Mechanics%20of%20Solids%20II.pdf

Whether to bolt or weld? If it can be welded that is preferable. If it cannot be welded then bolting is better than nothing. BUT, if the connector does not have a closed cross-section then it must be welded to gain rigidity. A box is many times stronger than a U shape. When you weld you use the floor as the fourth part of the box. If the factory was doing it they'd use a number of spotwelds along a flange. That would be less effective than welding, but more than nothing.

Look at your car realistically. Is it going to be a collector's item one day? Will you want to sell it? I'm asking if originality is REALL&Y that important. One thing for sure. Your car was designed at least 50 years ago and it was intended to be a grocery-getter. Also they were light. They were flexible.
My 1975 Mercedes sedan was designed at about the same time. If I put a jack under the front jack point right behind the front wheel and lift the tire off the ground, the rear wheel on that side is on the ground. I can't open or closer the door. On my 2007 Elantra, when I jack the front wheel at about the same jacking point, the rear wheel comes off very soon after the front wheel does. The door opens and shuts easily. That's the difference between new and old, being stiff torsionally or being floppy.

Except in 4WD jeeps, frame flex is not your friend.
R.

If they had been intended for all out drag racing they would have been built differently.

Every car I've added stiffness to has been better. Noticeably. Bolt in will help. As good as welded I dunno. You'd have to do back to back to say. My welded cars were wonderful though. I'd probably go that route

These points above you mentioned stand out to me. Depending on the application and user, I hesitate to categorically state what "we" are most concerned about, your last point is one very common example

Using common sense and intuition is a pretty effective useful way to start a path for a better solution, I don't want to discount it's part in the process, engineering will focus in on the best bang for optimum solution(s), with numbers.

Using a u channel SFC and having the fourth wall be the oem chassis floor, unfortunately accepts the shortcoming of that fourth wall being only 1/3? the material thickness of the other 3 walls.

Realistically the unibody chassis was much more rigid then the typical ladder frame back in the day, I think the biggest change is, our expectations have increased, not that the chassis was flexible as designed.

Other then that we agree pretty much.