Try second gear, E Brake smashed to the floor as far as it will go, jam a wheel chick on each side of a rear wheel, a piece of 2x4 will work as well. Breaker bar with a long pipe on it. If the engine still turns try third gear, if it still turns you probably have a clutch issue. You can also try heating just the bolt head for 30 seconds with a torch so the bolt length grows but don’t let the heat hit the balancer unless your planning on replacing it with an ATI.
An impact would be great but you’ll need to get the radiator out of the way to get one in there most likely. The fly wheel locking tool is the way to go though, If I had one I would send it to you and let you borrow it but I don’t have one maybe one of these nice guys might let you borrow or rent theirs?