I am finaly getting started working on the 73 Mach 1, 351 Cleveland 4V, that I traded for last fall. The first things are the basics, rebuilding the front suspension, brakes, steering, and fuel lines. I spent the winter tracking down original parts like valve covers, air breather, and correct exhaust manifolds. The plan is to put the stock exhaust system on the car from the engine back (turbo style muffler, maybe magnaflow). The supplier for Ky Mustang (I deal with a lot), will make the pipes in 2 1/4" (stock) or 2 1/2" at the same price. All of the charts I have looked at say 2 1/4" is good up to 400HP, and i dout I  will ever see 350HP (closer to 275-300), I know the 2 1/4" will fit better, does anyone see any reason to upsize to 2 1/2" (it may have a deeper sound).