If you do want to run it on propane make sure you put hardened stellite exhaust valve seats in to the heads. I wouldn't recommend running anything on propane, it burns hotter than gasoline and will wear out the internals of your engine faster. I would go with a 350, there is way more after market support and it will be cheaper to build than a 360. You can build either to have more hp/torque than the other but I don't think an ax-15 will hold up long to either of them if you go with more than a stock rebuild.