Back in the late 1970's I maintained a fleet of vehicles that ran on Propane. No GMC V6's in the bunch but a wide variety of other engines. You'll definitely need hard exhaust valve seats, as LP is a "dry" fuel which is very hard on seats. It's also 110 octane, so you can advance the ignition timing way beyond what's used on gasoline. If you're using evaporated gas through the throttle body (the most common type of LP system) you'll see a power loss due to the displacement of air by the fuel, usually about 10%-15% with no way to regain it. I have seen some attempts at injecting liquid LP below the throttle plates, but then there's a major problem with cooling the intake manifold due to evaporation that can build up a huge amount of ice inside and outside of the manifold runners.
Propane systems are interesting but there's a LOT of gremlins to deal with.
Jerry
