Raben,
Do a search on crankshafts on this forum, lots of good information on pros and cons of different year crankshafts, knife edging, etc.
Adding backpressure to an exhaust system is never a goal, but sizing the elements of the entire system to maintain flow sometimes appears to have that result. I don't know what you mean by cat-back; your 63 would never have had a catalytic converter. An x-crossover would be the most efficient, but won't be as effective on a straight six as on a V-8 as the six already has evenly spaced firing in each manifold and the firing order of the V-8 precludes this. For the street, just run whatever sounds good to you and run open headers without restriction on the strip.
Baldy
