I have used an old propane tank for an air tank as well, however I don't recall having much trouble removing the original valve. I replaced the valve with a couple of fittings and now have a gauge and a quick disconnect on the tank. The other thing I did was to drill a hole in the tank and add an automotive valve stem so I can fill the tank up anywhere.
My other propane tank project was to make a pressure blaster out of one. I bought the bottom strainer assembly from TP tools and inserted it where the old valve was. This now became the bottom of the blaster, I welded an axle across the tank collar and added some wheels. Then I welded a handle to the back and a leg to the front. Finally I cut a hole in what used to be the bottom of the tank, tapped it for 1 1/4 pipe. threaded in a short ******, welded that in place and then added a "T" with the branch bushed down to 1/2 inch pipe. I used this to connect the air supply and the bypass line that goes to the bottom of the tank. The top tapping of the "T" is fitted with a pipe plug with a handle attached and that's where I fill it from. I used some misc. hose and added a deadman valve from TP tools too. The thing works great. I seem to recall that all the parts cost about 75 bucks. This was many years ago and you can buy cheap Chinese blasters for 100-200 bucks now but with the parts from TP, the home made one is still probably a better machine.
BTW, a great way to make sure the tank is empty of propane is to simply fill it with water, then empty the water out and go to town!