2 - 2.25" pipe with "H" or "X" equalizer should be fine. If you run your pipes back between the rear frame rails and gas tank, then 2.5" is maximum. If you are planning on a stroker motor or 351 swap later, you might want to go a head and run 2.5" pipe now. Otherwise, I wouldn't bother. Remember that you exhaust will flow only as good as the most restrictive section of your system - that is usually your mufflers.
I have the same setup as you but without the cam. I have tri-ys to flowmaster 2.5" all the way back to 40 series. No problems at all. i got pics if youd like.
I would go with 2.5" max, because I think anything larger and you will start to lose backpressure with the amount of power you are making. I have 2.25" installed by the PO and would go with 2.5" if I was to ever redo the exhaust system.
My favorite JPG..lifted from the original source (long forgetton) but I still feel it is valid...even without the actual study to back up the conclusions
There are LOTTS of 5-600 hp cars running (dom) 2.5 duals... (ala: 03 cobra for example)
If you run crush bent (mufler shop system) then 2.5" is really only 2-2.25" worth of flow, because all the turns are smaller than they're supposed to be.
Some of the big turbo guys make like 700-1000 with a single 3.5 or 4" and split it to dual 3s for the cat back. So dual 3" is probably about as big as anyone would ever need to go, but thats jmho.