Bob,
The OD trans that you would probably want to put in your 71 would be a Saganaw which has a 1st gear syncro. It looks different from the early tranmissions as it has a 7 bolt side cover, the early ones have a 4 bolt side cover and are not syncro in 1st. They all use the same electrical/mechanical operating components.
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