I have done numerous 3 speed OD conversions for Stovebolt members and others. I have some rebuilt units available.
There are other parts that are recommended for the conversion to include a pull cable, relay, kick down switch, and wiring loom. If you are in the So Cal area I may be able to help you out.
I do not ship.
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