3 speed manual

[notahybrid]

I have a 68 c10. The truck has a 250 that has parts from varrious vehicles, ie. intake and carb from a Buick. The engine runs fine after a little tinkering however, my transmission pops out of R and 1st after rolling about 10ft. 2nd and 3rd seem to be driveable, also this does not seem to be a linkage issue nor does the floor panel interfere with the shifter operation (originally a column but was converted to floor). My question is, does anyone know of a 3 speed GM manual transmission that has a 4 bolt side cover???? I know this does not match the description of the warners, saginaws, or muncis of that era. I currently do not have the casting numbers but can provide if needed. I suspect that this is an older tranny that was slapped in prior to my ownership. Any info would be greatly appreciated.