I just happened upon this post by chance.

I had this same issue with my 1996 3.0 Aero years ago the smaller hose on that fitting isn't a return for the heater core it goes to a molded T in the lower radiator hose.

I believe it to be a coolant bypass so coolant can circulate through the block when the thermostat is closed but that's just a theory.

Just like yours the fitting that threads into the block had rusted through and had pinhole leak in it right at the hose barb.

When I went to the Ford dealer it didn't show up on any of their parts diagrams and I ended up making my own fitting out of a brass 3/8" Tee from Home Depot and a 3/8" threaded hose nipple from Dorman that I bought at the auto parts store.

Check out this thread I posted with pics.

https://www.ford-trucks.com/forums/4...-3-0-aero.html

Be sure to use RTV on the threads so you don't have any leaks.