The automatic used in 59 would probably be a dual range Hydramatic, or possibly a Powerglide. How many times does it shift? Also, the shifter quadrant on the steering column will give you a clue which one you've got. If the shift positions are P-N-D-L-R, you've got the Powerglide, which is a 2-speed. The 4-speed Hydramatic will have shifter positions of N-D-S-L-R, or possibly N, 1-4, 1-3, 1-2, and R. Reverse also serves as Park in the Hydramatic when the engine is not running.
Either way, there won't be a bolt-in fit for a later model transmission. The Hydramatic truck is a really rare bird, and it's worth a lot more in original condition than one that's been butchered up with an engine/trans swap. There are some unique parts like different crossmembers, and, I believe, a different size and shape hole in the floorboard for the Hydramatic trucks. Chevy trucks with the Chevrolet engine and the Hydro are very scarce and pretty collectible. GMC's with the V-8 engine and a Hydro used Pontiac engines, not Chevy. I'm not really sure if the 2-speed Powerglide was an option in the trucks in 59, but I don't think so. Be careful with your swap- - - -you might be destroying a very scarce truck that someone would love to have to do an original restoration on!
Jerry
