All sorted. It was silly simple - I just didn't understand how it came together until I took it apart.
What I didn't realize is that the compression fit collar is an NPT fitting that can be replaced by anything else that has an NPT thread - so it was just a matter of getting an NPT barb. One of the parts cars at our "race shop" already had the barb in its very-corroded tank. A couple new copper washers, a bit of a clean-up, & job done.
(To be honest, I'm not sure if it's actually NPT - I got the barb out of the same spot on the parts car, so I knew it was the correct thread)
old compression fitting on old hose: