Your solution is just great! Tri-fuel Tri-power!
With gasoline, you just over supply, and safeguard your exhast valves, pistons, and plugs. Extra funny gas is blended with extra fuel as per the jetting arrangements supplied with your kit. If you go overboard with a gasoline spike or augumentation, you can't hurt the engine at all.
With propane on its own, too much over supply will not give a cooling to the valves. You need a back stop to the system. Propane is a dry fuel, and the heat is a major issue if its not dealt with. your gasoline addition is just the capper!
Remembering LP Gas has no latent heat of evaporation, you'd need to spike the combustion process with a "water and methanol" water injection set up, if you weren't using 98 octane. The excess fuel factor, which requires extra fuel to stop the engine leaning out in a gasoline engine, is not an alternative which will provide cooling with a propane and nitrous on its own. When you richen the propane mix, it may cause burning in the exhast. So I'd look at adding a Vaccum Performance Valve (VPV), Fuel Control Valve (FCV) or Starter Valve (SV) to ensure you never reach a lean condition. Impco make these, and today there are excellent electronic modules to trigger them.
Adding gasoline is a better solution than getting either of these valves set up.
But sole LPGas +N20 has been done, WITHOUT GASOLINE AT ALL. I saw a 540 cube Donovan in a Cadillac Seville with nitrous, four E-series convertors, and four CA 425 carbs to ensure no lean-out occured. I've got a book on automotive fuels, and it covers the systems and theory involved in anything form Hydozene to digestor gas.
