[83GMCK2500]

Goto this thread: PDF of Square Schematics & GM Training Manuals (link)

Scroll ~1/3 of the way down to the parts catalogs on .pdf, look for Catalogs 51 & 51A.

On page 79 of 51A you'll find a picture and list of parts for the 1974-78 "CK" Gas Engine Oil Cooler & Related Parts. The numbers there reference the group number the parts are found in Catalog 51 under, can then find the part numbers.

I often flip through the parts catalogs to see what was/wasn't available. If you're not interested in 100% correct as the '74-'78 parts may be hard to find you've always got the later stuff to look for like gmachinz posted.