Had the same issue with 343 a couple of years back, not wanting to split cases and all, raised front end with ATV jack, removed rear plug, used a 7/8-16 tap set with generous amount of bearing grease in the flutes and sloooowly started enlarging the hole, very frequent tap removal/clean off grease and replace grease, didn't want ANY chips loose inside the cases, and after final cleaning installed new drain plug (NAPA part), so far so good, many thousands of miles and oil changes later....still running great!