i bought a 88 gmc sierra sle with a 350 engine, f.i., reg. cab., long bed. the previous owner said a three speed corvette trans. is in it. my question is, is the original trans. for this truck a th400 four speed?

 

A TH400 is a 3 speed you are thinking of a 700R4 4 speed. If your dash has a spot for overdrive than it had a 700R4 in it if it only has a 3 speed indicator than it came with either a TH350 or TH400. I am assuming the indicator is original.

 

ys the indicator has a spot for the overdrive. so it's the 700r4 trans? thanks.

 

That truck came with a 700R4. He may have installed the TH350 or TH400.