Reminds me of a project replacing the tankless coil on my furnace. Now, these bolts (no studs) have been in the cast iron for over 30 years. What's the chance that they come out without breaking them?

Well, I heated every one of the heads red hot and then went to town. No possibility of getting any penetrating oil in there because the heads are in the way. There are 14 bolts holding the plate in place.

With extreme patience, I removed 11 of them and broke 3.

With extreme patience, I drilled carefully on center and slowly increased the size of the drill up to the minor diameter of the thread. Bolts still would not release. Then, I managed to start the tap on the existing cast threads and cleaned out the remaining bolt threads.

What's my odds of doing all three? Managed to do it. No damage to the cast iron.

I cannot imaging trying this with an aluminum head, especially in the situation where you cannot position yourself directly in front of the hole. The odds of success are probably less than 10%.