I worked at a shop with the Schwan's truck account. They're all LPgas. Factory stock small blocks, trick is to ADVANCE THE IGNITION TIMING. Schwans would supply a kit, which had the specs in it, I believe (and don't quote me) base timing is upped to 24 degrees. This is instead of any internal mods. When done right, horsepower is still decent and they run fine.
Don't forget to run your fuel lockout solenoid thru a simple alarm/killswitch !
