There are currently some used / rebuildable GM OD units on Ebay.
https://www.ebay.com/itm/Manual-Tran...-/142666231532
https://www.ebay.com/itm/1955-55-Che...-/362229877350
How flexible are you with OD choices? Is a 4 speed with OD a possibility or are you sticking closer to the original configuration?