Your reson for changing your transimission seems a bit lacking. If you are really doing this because you need to drive it from Georgia to Colorado that seems insufficient to me. There were many (a good percentage of the over one million produced) 1966 full size cars with a 327 and a powerglide that drove much further than from Georgia to Colorado. There is no reason that a car with a good working 327 and a decent powerglide should not get you there just fine. It is probably less than 1500 miles.
I would worry about such things as cooling system, hoses, belts, tires, brakes, wheel bearings, etc. before I would mess with the transmission unless you feel that it is acting up.
If you do decide to change your tranny for a newer non-stock unit would suggest you put a few thousand miles on the car before you venture off on that long of a trip to make sure you have the bugs worked out.