/ 6 3 speed to a 4 ?

I doubt it. Only if your bellhousing has the dual bolt pattern on it, can you bolt a 4 speed on. You need a dual bolt pattern slant six bellhousing. I think the only way they came was with the 5.125" bearing retainer opening for the 4 speed overdrive. If you're not running an overdrive, you'll need a 5.125" front bearing retainer for your transmission. Not hard at all to find. I have a bellhousing if you're intersted. As long as the output shaft is the same size, the drive shaft will interchange, because the transmissions are the same length.