I dont have much for experience with propane, but ill give you what little I got. I think somewhere around 11 or 12:1 compression is a good starting place for natural aspirated (some run higher but ive heard these are reasonable and reliable numbers). Many things come into play when deciding on a compression ratio just like gasoline. Heads, cam/timing, alltitude, forced induction, fuel (even propane composition can vary from region to region,mainly butane content I think, but it effects the octane rating). One of the main things to remember is that propane does not have anywhere close to the cooling effect that gasoline has on the cylinder and valves. So premium valves and hardend seats in the heads are a must. valve seals may need to be different because of the extra heat and dry fuel gas. Ive also been told to shy away from aluminum heads because they can be too easily warped. Some run aluminum, but I personaly know of a few guys that are pulling the aluminum heads on their newer propane converted trucks after very few miles (dont know if they were tuned properly though?) Forced induction?? I know its going to add that much more heat. Obviously the CR will have to be adjusted accordingly to how much boost you will plan on running? Again, this is what little I know, hopefully I helped a bit.