The original transmission was probably a SM420,it was the most common choice. You should be able to find one from a '55-'66 Chevy/GMC 1/2-1-1/2 ton truck easily,and they'll fit equally well,though some bigger trucks had the E-brake on the transmission,so you might have to remove that stuff. If it had a 3 speed,there again,pretty near any GM 3 speed from '55 to '66 can fit with only slight mods. (Maybe a Saginaw 3 speed w/overdrive?) You're probably set up for an 11" clutch;if so an "LUK #04-064 12" clutch kit will fit and for some reason lists for less than the 11" kit.
Regarding the 265,they carried '55 Chevy cars to 100 mph and more,so it should be able to go as fast as you'd like to go. The 265 didn't have provisions for an oil filter,if I'm remembering right,so you might want to do a search for a conversion kit to remote mount one. Have fun with it,and ask lotsa questions.
Speed
