Vincent Flores

Ok, I have a 1990 chevy cheyenne 4.3 ltr v6 engine, with a getrag 290, 5 speed manual transmission with overdrive, I am wanting to switch out this transmission with an automatic transmission. I was wondering if a THM400 from the same year and make of vehicle would bolt right on, and what else i would have to change, i know i would have to change my rear driveshaft but is there anything else i would have to change (only if i can switch them out) If none of these trannys will fit(THM400 or THM350), what automatic transmission would fit?

rickpilgrim

Best fit would be a 700R4 as they were installed in those years so bolt up and wrecking yard driveshaft and go.
Now if you dislike the 700R4 you can do TH350 or 400, the holes may be there for the 400 as they were used in the 88-90 3/4 / 1 ton trucks but driveshaft will be custom due to U joint size difference. You can buy a kit to replace a 700R4 with a TH350 so I'm sure that will work also.
Do keep in mind that the 5 spd is an overdrive trans, and the TH350 and 400 are not so expect a fuel milage reduction. And it takes 10-35 hp to run an automatic also.
For the shifter you can either find a wrecking yard steering column with the shift linkage or go aftermarket shifter on floor, your choice.