Heh I just took about 3 dozen expired 20-pounders to the scrap yard. My old roommate used to work for a propane company and he'd grab expired ones from the dumps etc. from time to time. They'd re-valve and repaint them, and then sell them. Problem is he moved out a couple years ago and didn't take the stack of cylinders he had piled up out back. The only big propane tank I have is a huge round one, probably about 40" diameter. It would be awesome for a stationary large-shop compressor, but it would take up way too much room in my basement shop and is already probably pushing 300 lbs. on its own. I do have an auto propane dual-tank setup that was sitting out back when I bought the place, but I'm not sure if they would be much bigger than the pressure tank, or for that matter if they are even usable anymore. They've been sitting on the ground for heaven knows how long, they might be rusted right thru by now.
Backwood - the thought has crossed my mind to use a second set of wheels on angle rails at 90° to the top ones (so the "point" of the angle would be perfectly horizontal to the bed rails, not like an "L" as in your case). This way the top rails, set up like a "roof" as you described, would bear the brunt of the weight, and the side rails would help stabilize the mill by eliminating any lateral shifting on the top wheels, and would also hold the mill down so it couldn't bounce or climb up off the top rails (more useful on a portable mill than a stationary one). That can always be added later if necessary though.
