[P.J.]

Try using a small torch heating only the steel plug then use a snap on adapter for the removal of the GM plug on a johnson bar. Failing that and sometimes you can get it out
without ripping the threads right out of the intake.Then if it still does not come out get the plug drilled.
Drilling the plug out with the right diameter drill for a pipe thread. Yo must first centre drill the plug with a centre drill, then step drill with the proper dia of the
size needed to tap the pipe tread. It would be advisable
not to drill the plug by hand as that creates a larger hole
than necessary.See if you can get a machine shop to do it for you at a reasonable price.They can also use a plunging endmill on a Milling machine. Sometines its cheaper than buying a new intake or have the intake welded and then
reworked.
PJ