Really it's best to take a tape measure, check rear segment length to start with....when these things come up....

36" is the book spec length for later B body. All except 454/455 are shorter than 36".

In the case of my '69 it had to be 454/455, nothing else would work, same issue as original poster.

I believe some of it depends on how much the arch flattens out with vehicle weight applied (or does not flatten out
as in the case of an earlier poster).