Wait for it......another "what will it make?"

A 3" collector would certainly help, I have seen Pacemakers with a 3" collector near a real 500hp on various 360's. I ran what little data I you've supplied there through Pipemax and it spat out tri-y's at 1.737" x 18"-19" into a 1.862" x 18'-19" second segment with a 2.75"-3" 20" collector. (I assumed you had 10.5:1 and it would make peak power at 5500). If it was a 4 into 1 it would be 1 5/8" - 1 3/4" x 35.5" - 38.5" long with 3" - 3.25" x 20.4" collector.

Factory headed small blocks seem to love long collectors from what I have seen, I have seen instances of 20 hp, give or take from an incorrectly sized set of pipes. Pipes can be fickle on some engines unfortunately, pipes that should make big power haven't, and vice versa. Pipemax seems to get you pretty close.