Umm, I can't see ANY REASON why you need a 500 gpm pump on a brush truck. Even 250 gpm for that matter. We have a 500 gallon tank on our deuce, along with a gas powered pump. It has 2 small 1 3/4" lines, but we never use them because they dump the tank in a couple minutes. Your best bet is to use a 1" booster line on a reel, since this is easy and it stays round, regardless of if there is water in it or not. Put a fog nozzle on the end and you have the options to go from 5 gpm to 40 gpm. This works great for brush fires, and your tank will last you quite a while. Tiny little brush fires can be put out with 500 gallons of water easily with a booster line. Anything big, you need to make fire breaks, get helicopters/planes with water, set a backfire, etc.
Where I live is directly at the end of the funnel for brush fires in NJ. Our county burns the most out of all of them, and the way it moves, my town is the last thing for it to burn before it reaches the bay. It never makes it there though because NJ FFS is the best on the east coast.
Unless I'm misunderstanding you and you're trying to make this an engine, but I doubt that because you don't pump and roll with them.
P.S. If you don't already know, 50 gallons of foam costs a lot. It's $1,200 for a 55 gallon drum.