I believe most years of the 60's trucks (and cars)had the short tailshaft for the original 3 speed. The OD trans is the same length as the ones used in the TF trucks. You will need to modify your 66 driveshaft for this install but it doesn't need a rear mount.
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