Sounds like you might have a whole lot more corrosion than I'm used to seeing, but I want to post this trick just incase. Fasteners that are Known to be made of cheese (round out easily) are best treated gently if you want the best change of success. Throwing "mor powar!" into the equation just means throwing a welder or EDM into the mix too, more often than not. So try this instead (any stubborn fastener, ever) ~ apply just enough force that nothing's slipping and hold it there. Torque (force) is cumulative, so you either add a TON over a short time, or a Little over a long time. The downside to using lots, is that you risk rounding the means by which you have to rely to remove the fastener. It will absolutely take more time than you think is reasonable while holding the wrench, set a timer and don't let up for at Least 1 minute, more is better.
This is a simplified (and not technically accurate) statement, but easy to remember and Very functional.
Other tricks are to try Tightening then loosening. Apply some heat to Just the bolt. Give it a good smack with a hammer. Mix, match and combine.. I usually try time first, then tightening, then hitting and finally heat (can do the first 3 without moving from the work).
FWIW, all of my diff drain/fill ports were in good condition, fought me a little to come off but were removed without damage and I reused them all.
