I tried that Too....Bent the Screw driver shaft (twisted it)...it was really stuck...:D...I used a wood boring drill, small than the plug and came from under neath....as I almost got all of it out, it unscrewed itself with the bit...I didn't mess up any threads coming from underneath and using a slightly smaller bit....Used a wire bottle brush to dress up threads and will run a 3/4 in bolt in there with WD 40 to get them nice again. I'm just going to rebuild the whole castings...they were pretty old anyway. ......Thanks though.
Mark Milam