gotta go 3 inch

I had no trouble fitting dual 3s under my 68Barracuda. Springs are moved inboard with the 3/4" kit. I'm running lowered too. A good 2 inches or more low

Regarding backpressure.
http://www.dsmtuners.com/threads/exhaust-the-straight-scoop-on-backpressure.168578/
http://www.nsxprime.com/nsx-faq/exhaust-theory/