I just did this yesterday on a Saab(can I say that here?)
The exhaust studs broke off, oh, about 1/8 into the head. I build up a tower of weld.
Basically one bead on top of another till it protrudes above the surface then weld it to a 3/8 nut.
Dude, it might take a couple of tries but it works slicker than ----!
I got 3 done in 1/2 hour, no drilling. You got a mig? set the temp a little on the hot side aim for the center of the bolt and take your time.
Just a suggestion