Need your help on 383 Dart exhaust

I would take the parts he is giving you, get the Summit kit, and have a local muffler shop join the manifolds to the summit system.

Manifolds versus headers is the BIGGEST concern/obstacle with Big Block conversions in A bodies.

If you are comfortable with the manifolds for a while, then go for it, and if and when (once you see conversion header pricing) then you can have the muffler shop adjust the headpipes to connect to your existing system.