Just bought a 71 521 pickup a few weeks ago, and im having a hard time identifying what transmission the guy gave me. The truck originally had the original 1600 engine but that was a tired and beat old thing. Now i have a L20B from a 510 and its not mating with the trans. Would also like to know what 5 speed trans would work with the L20B, and what driveshaft would be needed as well.

(heres a pic of the unidentified trans) It also had these numbers stamped on it as well... 3X16289 Idk if they mean anything but just thought I'd post that too. Any help would be appreciated!!!