Re:
It needs to be a liquid valve, which basically means it has an internal pickup like a gas tank. This is why there is an orientation hole to lineup to make sure the tank is mounted properly, as the pick up is straight to the end of the tank then curves to one side, so you mount it in a way that it curves toward the bottom when laying horizontal.
100 lb is damn big. Mine are 10 gallon/43.5 lb I think, just a bit bigger than forklift bottles. Other than that, I don't know of any reason it would make a ****. I keep a standard grill bottle with me for a reserve tank. It will barely run mounted like normal since it's a vapor tank (has no pickup, just works off vapor emitted, as liquid propane gathers in the lowest point.), but I can turn it upside down and strap it down and it runs like a champ.
That big a tank, if it's not a liquid tank that has a pickup in it, you are shooting yourself in the foot unless you can mount it upside down. And of course the obvious, if the fitting matches up with your fuel line. I don't know **** about a tank with side fittings, but I couldn't imagine it being a liquid tank with a pickup and having the valve in the middle of the tank.