I worked at a Chevy dealer and a Cadillac dealer back between '83 and '85. As drivers, these engines were and are JUNK! Beware before sinking ANY money into making it drivable as these suffered from all kinds of problems. The biggest and most expense is the injector pump at the front of the engine. These were constantly being sent out for rebuilds (at $600 each in 1985 dollars). I saw camshafts go, blown pistons, etc.

I'd swap the mounts and put the gas engine in. It will be far more reliable.