Try shimming out your striker plate by about 1/8 inch. I know it doesn't make any sense but it worked for me. That link you were given only gives you a part of the manual's instructions for fitting your door... some important information has been left out like the amount of gap you're supposed to have between the striker and the latch which is no more than 3/16.
Get a copy of SERVICING CHEVROLET TRUCK CABS from The Filling Station. It's part number DG-62.
PICTURE 1 and
PICTURE 2
