I've got a '78 express and I took the oil pump off of it a few years ago since it was clogged with dirt. The gear inside the pump was stripped, so I bought a used one off the net.
the thing is, I'm not 100% sure that I have it together properly. I have the old one for comparison, but it's in pieces too. I have a fat (about .25 inch) shaft inside of the body with a gear the size of a quarter and there is a spring on it that presses against a funny piece. This piece is a fat cylinder with a skinny shaft that goes inside of the larger one, as far as I can tell. It has two "ramps" on it which I presume ride against the cover, which has a bump with a little flat spot on it that corresponds with this ramped piece.
Does anyone have any experience with these pumps? I'll take a picture if need be.