Most likely the valve cover leaking. When oil leaks at the plug holes, it will puddle around the plug until it's removed, then it runs down the threads of the plug and into the cyl.
It's not the main (outer) gasket that leaks, but the 4 smaller inner gasket/seals around the plugs holes.
Check for 80-82 F100 axles. Some of these trucks used the 4.5" bolt circle, and they all had a 9" axle.
If it wasn't pouring rain still (6" and counting in the last 3 days) I'd crawl under my 80 F150 to get an axle width measurement for you.