If you have more than one stud broken off, bite the bullet and remove the head. I've repaired one or two in the car, but removing the head is really the best way to fix it. Go ahead and replace ALL the studs with quality fasteners while it's off.
Back in the early 70's, I broke the last exhaust stud on cylinder #8 on a 383. Basically broke($) at the time, I removed the exhaust manifold, then welded a drill bit to a foot-long 1/4 rod; I used a hole saw to drill a 1" hole in the fender well directly across from the broken stud, drilled it out, ran a tap in it, and replaced the stud. Put a body plug in the fender hole, covered it with undercoat and I was back in business. Sold the car years later...ff you happen to have a red and black 68 Road Runner hardtop from NC with a funny hole in the passenger side fender well, now you know how it got there!!

The best way to do this in the car is with a right angle hammer and a left handed tappunch. Weilded properly the broken studs can easliy be driven through into the water jackets and later fished out through the radiator petcock like a kidney stone. Or, you could just pull the head. Entirely up to you.

i broke all 4 studs putting headers on my 318. i just drilled it out, stuck a magnet in the hole to get out all the excess metal, and re threaded it. i had to cut my inner fender wells to do it but it was an easy fix with some sheet metal and pop rivets and actually looks kinda cool.

pull the head. take it to a shop and have them do it. It will save you much time and hate