4 on the floor or console auto?

Well, what gears are you running in your car, what height tire, what is the car for?

If you have a 64 4 speed does it have the trunion tail shaft or the slip yoke and what is the part number of the trans so we can get the actual gear ratios?

Considering your rear end ratio, purpose for the car and the gear ratios of the transmission and your engine build (purpose of car) will help answer your question.

How about answering those questions and then you will have your answer on whether to switch. Personally if I were running a small block I would most likely go wide ratio 4 speed with 2.94 or 3.23 rear ratio and 26" rear tires (if it were a street car that needed to have a reasonable cruise rpm and lots of fun).

For example, if you have a 64 4 speed trans with the fixed tail shaft it likely has a 3.09 first gear which allows a higher / taller rear gear ratio which will allow for quick take offs and still allow for low rpm cruise speeds.