Is there anything left of the old mufflers ? If so look through them and see if there are baffles inside. If not, a straight pipe will work fine.

Since originality isn't an issue, you can look up a universal muffler that will fit. Walker has a look up at https://www.walkerexhaust.com/universal-muffler-search.html. I'm sure there are others. There are also universal bends and elbows available.

If you are near an old school auto parts store where they still have physical parts books they can show you dimensioned drawings of a large range of parts. They might even have a listing for the rear muffler.

The difference in performance in any case will not be noticeable.