I had trouble with K10 shocks a couple of years ago. 3/4 inch mounts were hard to find unless you were driving a tractor trailer. Found these part numbers and bought on Ebay. I can't vouch for the length and compression differences between K10 and K20, but at least a start for your search.
508-30
22066254


~ HB
1966 Chevrolet K-10 | Ghost: formerly Flappy Fenders | In the Stovebolt Gallery
1962 Chevrolet C10
1962 Suburban