Both my trucks run 351Cs with duel fuel setups eg.petrol and gas. The 100 I run exclusively on propane(we call this gas out here) now because the propane has wrecked the carb(750 holley) causing it to run rich on petrol, the 350 I run about 4 tanks of propane then 1 of petrol to keep everything working(350 has new 600 Holley), the cost is a big factor out here, even though you do loose some economy and a little power on gas it is only 70c a litre whereas petrol is $1.40 a litre. As far as valve seat recession goes both my trucks have got stellite valve seats and stainless valves to prevent or lessen this, some people still like to run a flashlube top end lubricator kit on lpg but I don't have this on either of mine although the 350 has previously had it. Ive never heard that lpg can damage aluminium heads any more than it can to iron but I know that no one in oz will put lpg onto an LS1 but I heard this was more to do with the possibility of an intake backfire bursting the plastic intake manifold but it may also have something to do with the heads. On another note Holden(GM in oz) have an alloy V6 which comes with a propane option from the factory as do ford whose 4L six has an alloy head and they are also available as dedicated propane burners from the factory.