Like jon1963 said, it's probably the SM318. If it has 4 side cover bolts, that's what it is. The most common replacement would be the Saginaw 3 speed, used from '66 up. It uses the same input shaft, but the output shaft is a fine spline like a TH350 or 700R4 automatic. It is also a little longer than the SM318, so the driveshaft would have to be shortened or replaced. Also, the front bearing retainer on '68 up truck transmissions is larger, so to use one from a '68 or newer truck you could either use a bearing retainer from a car Saginaw or just have the truck retainer turned down to fit your bell housing. You could also use a '68-'72 truck bell housing. A car Saginaw would fit your bell housing, you would just have to use longer bolts with nuts for the 2 bottom bolts, the 2 lower bolt holes on a car trans are not threaded. Hope that helps.
