it will fit the engine in the 80 4WD
but you will have no working 4wd system.the 76 is a 10 spline and all the 4WDs 1979/1987 are 26 spline
if memory serves right......if "bolth autos" are short bed, you could put the 10 spline right in to the 80 truck with the 76 drive line.
but if the 76 is a long bed, the drive line will not bolt in to a short bed, it would need to be shortened.
also, if theres a frame length change 76 to 80 this would also make the drive line not bolt right in.
are you looking for a 4wd trans and can not find/afford one? or wanting to make the 4wd a 2wd auto
