If you had a TH400 in an '88 C1500 I'd imagine someone put it in there after the fact cause most half tons never came with a trans that heavy duty (especially in the later years)
It was factory. SWB 2wd STD cab 305tbi/TH400. Don't know how or why, but it was. Had to put a 2nd gear accumulator piston in it. They came with a plastic one and if you geared down manually, it broke them after a couple times. Our transmission guy had an aluminum one laying around I stuck in it. Old transmission is still kicking it. Traded the truck to one of Dads friends for the 89 t-top Camaro in my "other rides" thread. It was a doll of a little pickup.

