The c5 is not as good a choice as the C4- it was used behind 4 and 6 cyls, so not quite as strong. The C4 will be your easiest and they're cheap to find/build. Maybe sell the 5 and use the $ for a 4. Or, if you're looking for highway gearing or mpg, why not go with an AOD and get the overdrive? I did in a 65 and love it! As far as the flywheel, the weaight is pertaining to the engine, not the trans- so use the one you have on the engine as long as the tooth count is correct. good luck
You must be thinking of the C3. The difference between the C4/C5 is mainly that the C5 has a lockup torque converter function. The case also has some oiling improvements also. Install the valve body, input shaft, and torque converter from a C4 and you will be good to go. Changing those three items does away with the lockup torque converter function.