Dual Exhaust with 318 Manifolds

I believe the opening on the stock 318 manifolds is 1 7/8". You can put 2" pipe at the manifold, but that's as large as will fit. From there, you have to come out and adapt up to whatever size you want. I'd just run the 1 7/8" off the manifold and then increase the size. The difference between the 1 7/8 and 2" right off the manifold will never, ever be noticed. If you want 2.5" then you need some 340 manifolds or headers.