Dave - I found the same issue as Bill reported, with the part identified as GM# 6107373 - Fitting, oil handler (picture from the Book of Parts I found it in below).
Knowing this was a component of a GMC 302 which Uncle Sam bought lots of, I figured it's most likely still listed in Mil Spec records. A web search turned up the below data on the basic Mil Specs for GM# 6107373 and the refined specs of the same part under the National Stock Number assigned to it (collected from multiple sources). If you can decipher the details in the specs, I hope you can figure out what you need to replace your fitting.
If not, I'd suggest you post an ad in
Truck Parts Wanted. There may be someone with an old scrap block that would give up a fitting?
