i just recently purchased a 1956 pontiac chieftain 2 dr post.this is my first attempt at totally rebuilding a car, does anyone know of a late model car or truck that the chassis could be swapped out for with some modification? the car was built by an older guy who never finished it and the original frame is badly hacked,any help is greatly appreciated.

You are probably better off finding another original '56 frame.
Try for a truly rusty parts car, cheap, where the frame is still good.
GM cars of '55,'56,57 from other divisions would have suitable frames too, so long as the wheelbase is the same.
Swapping in another frame from a different model involves a LOT of work, including getting all the mounts and mechanical components to work.

Another alternative is to find a good welder/metal fabricator. Frames are often straightforward to repair.Even if the damage looks bad, it may be much easier to fix it in place, instead of swapping.

thanks for the help.this is my first time trying to take a car down to the frame and back again.