Not sure how easy it is to access the fill-hole on a 6 versus a Spitfire. But a common way to drain the diff is to snake a tube into the diff through the filler hole, and suck the gear oil out with an oil suction gun like this one from Napa:
https://www.napaonline.com/MasterPages/NO...ion=Suction+Gun
It is a real pain. You need to get the siphon hose in past (underneath) the crown wheel, which is mounted just inside the filler-hole, and it is definitely going to take more than one 'suck' to get the thick gear oil out. Actually you probably won't get it all out.
But its probably the least amount of work overall.
I'm not sure how you get the back half of the differential case open or off without dropping the differential.
As posted here, if you want to drop the diff, open and clean it and put a drain plug in it, that works. But its a lot of effort. I have seen posts here where others say you can actually drill a hole up into the bottom of the diff while its in the car (which certainly results in the oil coming out). And then tap an NPT threaded hole for a drain plug. I suppose if you were carefull not to drill too far and/or get chips into the diff that would work.
I have put a drain plug in my Spit. I needed to remove the diff for other reasons. But given the amount of times you need to change the gear oil, I'm not sure I would have done that if I didn't have the diff out already.