One thing you did not mention was what transmission is currently in your 50 and if you are happy with its cruising or acceleration. If you currently have a PG, the cruise will be the same and acceleration better. If a 3-speed, compare the ratios. There are several on-line calculators that will let you play with trans ratios, rear end ratios and tire diameters to help you determine where you want to be. I put a 700R4 behind a 292 because I wanted the overdrive due to a 4.10 rear and 26" tall tires. Just figure out what you want the car to do and plan accordingly.
On the 200 or 700, remember if you use the lock up converter, there will be some wiring and it is all 12 volt.
