I agree with Henry.

1 7/8" primaries into 3 1/2" collectors into 2 1/2" pipes, muffs, short tailpipes work very well on the 462" Buick in my 32 roadster.

In fact this combo was recommended by Kenne-Bell, one of the few late Buick parts suppliers and runners.

Generally you can take a cue from exhaust port size.