After taking the rear chunk cover off to do a fluid change, I realized the rubber plug was beginning to deteriorate and needed replaced. So I opted to have the unthreaded bung cut out and replaced with the threaded bung from a D60. Originally we thought it needed to be cut out with a plasma, but then realized a hole saw would work just fine.
Untouched original cover
Rubber plug on left. Plasma cut threaded bung on right.
Interior view untouched cover.
Interior view threaded bung welded to outside of cover.
Finished product. I'm not grinding anymore. I don't want to compromise the integrity of the welds.
3/4" tapered pipe thread plug with a 9/16" sunken hex head.