I wouldn't mess with the head shims till you run it. Usually, If you need a shim you can hear the engine pre-detonating. It's hard to explain the sound. Almost like a sizzling sound. How many head shims you run has a lot to do with what climate you're running in. It's possible you were running too many already, and switching fuels will put you right where you need to be. Just make sure you don't start to see pitting on the top of the piston, thats a sign of pre-detonation.