My riser bolts were a total pia.... but they were not allen type head (standard hex).
3 out of 5 brok off at the head but once I removed the risers I was able to get to the remaining parts. I used PB, Time, hammer, and a tighten, loosen, tighten, loosen technique. (Sounds like you are doing the whole manafold?)
Took me about 2 hours total time but they sat while working on other stuff soaking across several weekends...
Best of luck