you can get steel or aluminum NPT/AN fittings, then an AN 4 cap, at just about any online race supply place. Be sure you know if your rear end cover is steel or aluminum, so you can properly have it drilled then welded with the matching metal.
I use Redline for my engine, BG Synchroshift II in my Tremec, and Mobil 1 in my diff. Castrol SRF in the brakes, and Valvoline
Synthetic brake fluid (not silicone) in the clutch fluid. Use a huge NASCAR no-bypass
oil filter, on a mount from Ernie Elliot, off of one of the Evernham Dodge cars. Expensive, come sealed to avoid tampering.
I change them often. Works for me, over the past 120+ thousand miles, including over 3000 track miles.
By the way, it will use a quart of
oil in about 100 track miles, but goes about 900-1000+ miles per quart on the street.
all the best,