with my engine combo, it should be making around 500hp when its all dialed in, would single 3" be ok or 3.5" be a better route to go?

 

3'' would be best 3.5 would be too much and not build enough back pressure.

 

Uh, no. Even a 3.5 system will build backpressure.
Besides, you don't want backpressure anyway.

We need to stop beleiving in the old myth.

 

i know back pressure is bad, and the term i believe people mean to say is scavenging. i just dont know but ill prolly just go with 3.5"

 

Single 3 inch is not enough. Heck, I'd go for single 4 inch instead of 3.5

 

The 3.5" is all you need for 500HP. Tests have been done at Magnaflow to support this. Like was said above you do not want back pressure. By the way what do those heads flow? What is your dynamic compression ratio?

One more thing get the Magnaflow 3 1/2" muffler. It is of the straight through design and has very little back pressure.

 

Heads flow around 260cfm @ .500 lift and dynamic compression is about 8.5:1 if i remember correctly, and i already plan to run a straight through borla muffler (XR-1) where the cat would be and still run my spintech out back 3.5" in dual 3" out.

 

It will be tough to make 500hp at the crank but it sounds like you have a combination that might do it if built correctly. Trying to get close to that goal myself but with a 355 but that might be a little to much for me.