I have a pretty much identical setup except i'm using the normal Performer, not RPM.
I opted for the 2.5" system with X to retain some low end, but with a 5speed box, there is plenty of gear there so getting it to rev isn't hard either.
Unless you're in the 500+HP range, I wouldn't bother with 3". Tighter clearances getting the pipes over the diff etc..
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'71 Holden HQ Monaro - 3850lbs race weight, 400c/i - 11.4 @ 120
'66 Pontiac GTO - 389, 4 speed street cruiser
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