As you probably know, your '64 four speed will have a ball & trunnion output shaft on it. And while your current driveshaft also has a ball and trunnion front joint, coming from an automatic (which is longer than the four speed) it will be far too short to use. You're probably going to strike out trying to get anybody to build you a ball & trunnion driveshaft in this day and age (not to mention a severe lack of new/rebuild parts for the B&T), so my advice would be to have a new shaft built as per this article:
Early Valiant Barracuda Club - ball and trunion U-joint conversion
