Hi, welcome... Probably the best thing is to swap out the entire driveline, you can use a car or truck 3 speed, 3 speed with Borg Waneroverdrive if you can find one, a car 4 speed, one of those rare new Process 80s chevy truck 4 speed overdrives, or a T5 5 speed. For the rearend look for a 55-57 chevy car, 70s Camaro/Firebird/Nova, or some S-10s.
squirrel is right about the best way. But, long ago I did some late 6 and v8 swaps into 49-54 chevs using the pickup trans case from those years. It has the same bolt pattern, Main Bearing Retainer size and Main drive gear dimensions as the 55 up GM stuff. And all the car gears with fit right into it. You could start with an engine, motor mounts and this hybrid trans and do the rest later if you want to spread the cost and work out a while.