Sorry man, stay with your dad on this one... 3" exhaust will sound cool (LOUDER), but horsepower wise and drivability wise stay with the 2.5 max. 40 flows would be a preferance because they do sound awesome outside the car, but the resonance inside the car is a bit much on long drives. (read headache)
Your not needing anymore on your exhaust system unless you start near the 351 stroker range like 393,408,etc. Even then your not really using all of it. Really its HP Big Block territory.
If you really think about it 1 5/8 are abit over the top for your build. 1 1/2 would give you better throttle response.

I know how hard it is to read forums, Magazines, listening to friends, and not want the Hardcore parts but realisticly some of the best performing setups are "boring looking". "Yah ive seen that setup in countless articles."
But whats really important? When you step on the gas, it flys!
Good luck and have fun.

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71 Pinto V8 lifelong project.