The problem with the carriage bolts is that over the years the square under the head and the square hole in the front engine plate wear, and eventually won't keep the bolt from turning. The length has to be just right to get the bolt in with the crank pulley in place. I suppose a guy could pull the balancer off and get to the bolt heads or even just tack weld the head to the plate, although that's kind of a drastic measure. But then again, how many times will you need to take those bolts out. Another approach might be to weld some tabs on the plate to capture the flats on the head of a hex bolt, but that might eventually have the same issue as the carriage bolts.
In defense of the designers, they weren't necessarily worried about it working for 70 years. My daily driver truck has some bolts with "wings" attached to the bolt head which work at least once during assembly when threads are clean, but I've had to deal with a couple of them when working on the truck.
What bartamos said about fire. Go in hot and quick and cut it off. But like I said, the two inner bolts don't need to be removed to get the engine lifted. If you don't have a radiator in the way, you could probably cut the mount loose with a sawzall.
