I bet it was the center bolt on the exhaust side right? When this happened to me I got a drill bit that was about the same size as the shaft of the head bolt and drilled down into the hex head. The idea is to use a bit big enough to cut the head off the bolt, but not much bigger. I used rags and a magnet to catch the shavings. It took a big 1/2" drill but pretty soon I had a headless head bolt. The rest of the head bolts came out without issue, and the head lifted off over the headless bolt.

Then with the head out of the way and all the tension off the bolt I could tap it on the side a few times with a hammer to loosen it up. It turned out with just vice grips and a few taps. I originally stripped it out using a t-handle with pipe on either end and the help of a friend trying to break the bolt loose, but without all the stress on it it came right out. If it had not come out, I was expecting to use the torch to heat it up