I have built a few engine on pure gas. I wouldn't go higher than 11:1. Retime the dizzy, special cam grind. I just like LPG, because it's cleaner, no moving parts, no fuel pumps, crank, airflow, throttle position, map sencors etc. Also If you don't drive it much, the fuel won't go of and attract moisture etc.
Oil stays cleaner, plugs don't foul. I just don't like the smell. And with a 4 barrel manifold and a 425 Impco mixer and a big aircleaner, it'll look like a carby engine. If done right, it's very reliable.
JD