Your 64 probably has the short tailshaft trans which is about 15" long. Both OD transmissions are 22" long. In this instance you will have to modify the driveshaft. The later Saganaw unit will require a fine spline front yoke and some tweaking of the shifter rods. Either one would work fine in your truck with the 3:73 gears.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine