X pipe H pipe or side by side

H pipe - Lower end torque/power, deeper sound. The pulses will "run" into each other in the pipe. Helping reducing resonance. Think old school V8.

X pipe - High end gains. More horsepower overall. The pulses will "merge" better in the pipe. Causing a scavenging effect at higher RPMs. Think raspy newer high performance engines.

You can watch some shitstangs rev and compare x vs h in this video:
[ame="https://www.youtube.com/watch?v=lnI53sWqbcs"]X pipe vs H pipe REVS- Mustang GT - YouTube[/ame]

As for pipe size. 3". Don't even think of 2.5 at that kinda power. You'll choke it for sure. 2.5" is for around 400 horse. Read this: http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/ The math holds up. 3" would be great on your setup.