Another way. GM rear Diff

Welp I finally got around to installing my limited slip diff in the rear of my chevy silverado 1500. Installed new carrier bearings and slapped the unit in, after checks we found everything looked good, back lash was good, mesh pattern looked great, all`s a go right? wrong. I placed the rear cover back on and tightened the bolts. Next, install the royal purple gear lube, easiest part of the job. I get the ratchet and extension only to find the last person who put the drain plug in stripped it, so my extension just flopped around in the hole. So I went to plan B, heat it with a propane torch, then hit it with white lube so it would melt and work it`s way into the threads then use a chisel to loosen it up. So I get it cherry red and hit it with the lube then use the chisel and the thing won`t move. Heat it some more then lube it more, try the chisel again, no go. Finally after fooling with it a whole hour in 90 degree heat I had enough. I popped the vent tube off, got a piece of vacuum line and installed the fluid through the vent. The Diff holds right at 2 bottles of gear oil so I know it`s full. However, does anyone have any ideas how to get the blasted drain plug out? I don`t know what else to do other than drill it and that would mean I would have to remove the rear cover and install fresh lube again to get the metal chips out and Royal Purple is $16 bucks a bottle so I really don`t want to have to do that again. Any suggestions is greatly appreciated.