There is NEVER excess alternator energy.Why not just augment what you have with a HHO generator? If your preferance is for bolt on, consider a supergen. Check it out on Youtube.
Use the excess alternator energy to produce electrolysis, just like a car battery.
The one and only local place I've priced has propane at $3.30/gallon, and gas is at $4.30 a gallon.Why go to propane when gas is only slightly higher? You know propane will go up along with gas eventually. It would seem like a poor investment although I don't know how much a conversion would cost you.