71 Twister with 340 4-spd, Bent front frame rails
no they aren't all the same. I know the bolt holes for front bumper braces moved.
I'm not sure what other differences you might discover. Hope this helps
Yes, the bolt holes for the bumper brackets moved on the 75/76 cars. If you're capable of swapping the frame rails, this is an easily surmountable problem.
There are a few minor holes/brackets that moved over the entire run. The dimensions of the rails themselves the same, the suspension mounting points are the same, the attachment points to the body are the same. The only reason to limit yourself to a specific year or year range would be if you're planning a concourse restoration. Even then, the differences are minor enough to be dealt with.
As far as sectioning the rails vs replacing them- I know it seems like sectioning them might be easier, but if you have to go back past the suspension mounting points it really isn't. The rails are just spot welded to the rest of the car- the floor, the inner fenders, the torsion bar cross member, radiator support, etc. That is a lot of spot welds, but all of those structures also do a great job of locating the frame rail in the correct spot. And they're just spot welds.
The problem with sectioning it that you still have to maintain all of the geometry. And, you may actually have to put down quite a bit more weld, so you can run the risk of warping the rails. Keep in mind the frame rails are not really all that thick, just a step up from sheet metal really. It also depends on where your doing the section, because the frame rails are not just hollow boxes. There is internal bracing and multiple layers depending on where your section extends, and not all of it will be easy to get to because of overlap with some of those other structures.
If the section is small enough, ie, it's just the stubs ahead of the suspension mounts, then sectioning would be easier. If the suspension mounting points are involved, it's easier to locate the entire rail to keep the geometry right. It's also easier to spot the sections, so if resale is a concern I would swap the rails. From the pictures you posted, it's pretty obvious the rails have had work done already, and it looks like the suspension is involved.