Stringpickin,
In my opinion, the most critical area that affects torque, with regard to exhaust systems, is the first few feet outside the exhaust ports. For a 383 at that level of performance I would think a good pair of 1 5/8" i.d. tube full length headers to a 2 1/2" i.d. dual exhaust that has a cross over ( X pipe not H), around the end of the tranny. For best flow I would follow that X pipe with a true 2 1/2" dual exhaust out the back or wherever you choose to exit. Most performance exhaust pro's say when theres a question about header tube diameter " larger is not always better, and go with the smaller tube when you are in between sizes". Of course, the final application or intended use of the vehicle will play a part in determining header size. I.E. drag racing, circle track, street killer
, or just daily cruiser
72