Re: Questions on 2.5" vs 3" Exhaust
The Flowmaster set looks like something that could be made to fit pretty good.
Also, not sure if you thought about adding an X/H, but we just built nearly identical 427 sbc's (engine dyno'd at 630hp), one with duals and one with an X (from Torque Tech). The X has a much different (smoother/mellower) sound but I think the performance (based on 1/4 mile times/speeds) seems almost the same. The separate duals come off easily, but the X is a lot more tricky.
I think I'd do true duals if the header collector, extension, and exhaust pipe diameters were all good for the combo. Maybe the X/H helps when something in the system is restricting things?
69 Chevelle Malibu
427 sbc, TH400, 3.55 12-bolt
n/a best: 11.24sec/118mph