Im changing motor in my 85Firebird.It has hedman shorties ,2.5 y pipe to 3 ' pipe with flowmaster and 3 ' tailpipes motor makes around 325.New motors makes about 450 .Wondering if exhaust will be ok or go to 4 " pipe.

 

3" should be ok with a high flow muffler (read straight through design) but your closer to requiring a 3.5". 4" would be abit overkill

 

I don't know if the shortie hedders will make much of a difference, but I'm having a hard time running Longtubes into a 3" Y-pipe under my tranny crossmember without hitting the crappy roads around here....

Good luck!
(post pics when your done...)

 

My vote is for a 3.5" system.