Hum... an auto tranny. Uncommon, and much-maligned.
Consider this, if you stud bolt broke a head bolt could break too... Yeah, it's very unlikely, but pulling the head (IMHO) is a last-resort kind of solution. Avoid going to that extreme

Anyway, did you pick up some bolt extractors? long, kinda narrow high carbon steel things with a reversed bit on the tip? As I understand if you insert it into the hole and hammer it into the end of the busted stud, it will bite into the end of the broken stud and give you the leverage you need to crank it out. You then attach a socket wrench or similar tool and back it out of the hole. They make some hammer driver tools that do just this.

Even if your dipstick is in the way, you must have some access as the manifold used to sit there. A good botl extractor will be pretty short and won't need you to drill a hole first.

Very important. Do not waste your time with WD40. Don't get me wrong, it is great stuff, but in this case you should use P'Blaster (yellow can, blue cap). It's available a lot of places. Soak it really good and then wait a bit. Failing that get out a propane torch (like you use to sweat copper plumbing) and use some heat, followed by fast cooling with water. The stud should free up very fast that way.
--
1967 P220 Amazon, 1972 145S, 1976 245 DL, 1983 245 DL, 1986 745 GLE, 1990 745 GL, 1995 945....
You mean to tell me that Volvo makes cars that are *NOT* Wagons?!?
1971 P1800E... Not a wagon, but it's just a donor car for the Amazon..