editing in progress***
quench (or squish) is the distance from piston at TDC to head.
it's the perfect size air bubble in your compression chamber. when you squish that air bubble JUSSSST RIGHT it sends a shockwave (think of the *NOISE* of a balloon popping) down the combustion chamber that helps even the temperature of the block. i believe it has to do with resonance (like VRIS in an intake manifold)
BUT!!!
you are getting WAY too involved in engine building specifics for what you are actually considering doing. if you are seriously going to run one OHGX450... then you're not building a frickin race motor. so as long as your compression ratio is north of 10:1 you don't need to concern yourself too much with all that effective ratio stuff.
1:accepted optimal CR for LPG is around 11:1
2: squish, swirl, valve steps, shrouding, wet-flow, and turbulence can pretty much be thrown out the freakin window... you don't need to vaporize your gasoline between the float bowl and the piston. your Pane IS vaporized, and at 11:1 your mixer *tuning* and spark advance will control detonation very well.
3: you should be most concerned with a complete combustion event and opt for dished pistons to facilitate. i STRONGLY suggest having your (hypertectec) pistons, chambers and valves pollished. all those reflective surfaces will bounce that heat right out your exhaust. *side note- polishing all these things averages a 6% increase in HP.
4: 160* thermostat at the highest... make sure you have a glycol%, radiator, and enough CFM from your fan(s). some things (besides all the polishing) to help keep your temp down : eliminate EGR. use an aluminum intake like performer RPM that has eliminated the heat cross-over.
here's some clear answers for your pane questions...
the effective octane rating of LPG is 103 and you will get similar *performance* using LPG vs. gasoline
BUT it has a lower energy content (Propane contains about 84,000 Btu/gallon and regular gasoline averages 114,000 Btu/gallon)
so your *mpg* will drop, depending on load. at optimal load you will lose .08% MPG... but, once you put that pedal down to pass a blue-hair, or climb an obstacle, your MPG is going to drop much faster than .08%
if you're running forklift tanks, you are choosing between 10 and 16 gallon tanks. you're not getting very far with that. those tanks cost you between 100-200$ each.... how many do you want to buy and carry around?
but i keep coming back to the initial remark i made...
why on earth would you run ONE X-450 on a 472 when most% of people aren't happy with ONE X-450 on a 350 with decent HP #s!?!?!?
the ohgx450 is rated at 435CFM... can you IMAGINE putting a carb like that on a bigblock? granted, cfm means different things for gasoline/lpg...
you need to figure out your engines cfm demand at a given rpm... if it's ever OVER 435, then you are going to SUFERRRRRRRR anywhere faster than that RPM. do that, and i'm sure you'll see why you should go with a dual mixer setup. they will of course be smaller mixers, probably giving you ~300cfm each.