Can't change trans oil...Suggestons?

Well I gathered everything I thought I needed to make the change over to Swepco in my '83 SC. I got the 17mm Allen, the Swepco, the gear oil pump and the cheap oil to run in the gearbox for a short period of time.

I was able, with considerable effort, to loosen the drain plug. Luckily, at this point, seeing how difficult it was to get loose, I did not remove it. Instead, I moved on to removing filler plug. I don't know if it was more stuck than the drain or if it just has more difficult access but I stripped it (the filler plug)in short order. Being at least tenacious, if not brilliant, I simply moved on to bigger artillary. I got out my largest vice-grip and tried to get the filler plug out by gripping it by the exposed outer threads. The plug being soft alloy, this scheme didn't work either as the plug just started to disintegrate.

If any one has ever run into this situation and can offer any suggestions I would love to hear them!

Jess
'83 SC (with stuck filler plug)