Technical Ford 9" rear axle

I worked at a drag car building shop in Kentucky for years and we regularly built 9-in rear ends. We had a specific hefty trash bag that we kept rags in that were used to swab out rear ends. We would wrap the rags around a broom handle, just like cleaning out a revolutionary war cannon , swab the axle tubes out, then throw them in the hefty bag.
The shop owner was a good ole hillbilly, so we didn't have a rag service and once every couple of months, we'd go to a college dive in Cincinnati called "sudsy lemons". It was a laundromat with a bar and often had live bands. We'd stroll in there with two large hefty trash bags full of dripping, burnt rear end gear oil, dump them into one of their "industrial machines" and wash them twice on hot (twice). We'd then dry them In a machine that I could have fit in and I'm 6'6... By the time we left, the entire place, which was rather large smelled like burn gear oil and we were beer drunk and nobody ever thought to say a word.

 

The fill plug's hole is 5/16" I believe, and I do have one in my 57's trunk tool box......easy to make, just grind a little off a cheap 3/8 extension.
My OEM differential was shortened, changed to Yukon 3.50 trac loc, 31 spline axles, big late ends, etc. I found out it does not like synthetic. Only conventional 90W.