Exhaust size?

Your butt dyno will not be able to tell the difference between the 2 1/4 and 2 1/2. I would just install the headers and down the road if the money presents itself or the exhaust needs replacing go 2 1/2". The only thing 3" will do for you is make more noise.

My Barracuda orignally had a \6 with 6 into 2 headers. My ex father inlaw bent up a 2 1/4" dual system with an H-pipe for it at his muffler shop. When I swapped in the 360 (~370 HP) I just adapted the 2 1/4" system to the headers on the 360. My daughter was working for her grandfather at the muffler shop and offered to do a new system for me as a fathers day gift. I had her do a 2 1/2" system with an x-pipe and 50 series flowmaster mufflers. I could not tell any difference by the but dyno in the power between the two systems.