Propane will lay in the bottom and might take quite awhile to leach out of the tank, propane is heavier than air.
Just turn the tank upside down.
SE WI. I'd be looking to do either 80 or 120 for first smoker. Maybe am better off finding an air compressor.
After seeing "Mayberry" in your forum name I asked about your location as we live in the land of Andy Griffith in Pinnacle, NC. We're four miles from Pilot Mountain (AKA: Mount Pilot) and a few more miles from Mount Airy, NC, (AKA: Mayberry). If you'd have been local I'd have been glad to help you cut your tank. Heck, I have some well vented and ready-to-cut tanks in the woodline awaiting conversion and you could have had one of them.
Personally, I'd make sure a propane tank is vented and empty of pressurized propane before picking it up and after bringing it home I'd dump in a big bottle of Dawn, fill it up and wait a few weeks.
A stiff dose of Dawn dish washing detergent in the tank left to sit for a few weeks will tame the ethyl mercaptan so don't worry too much about that.
For what it's worth, a lot of knowledgeable folks think that 24" diameter is about the smallest that fits a truly useful second rack. It's also worth noting that air compressor tanks trend a good bit thinner than propane tanks so they may be a bit harder to hold steady temperatures in, particularly on cold or windy days. I know that propane tanks are hard to come by in some areas and we have to use what we can get but before choosing a compressor tank I'd hold out for a propane tank if possible.
A lot of vertical 120 gallon tanks (typically 4'6" tall and 30" in diameter) have been made into top notch smokers but my personal preference would be to try to find a horizontal 120 gallon tank (typically 5'8" long and 24" in diameter). That will be harder to find but, to my mind, it's a better shape for a mid size horizontal smoker than an upright 120 gallon tank .
If you have anyone around you specializing in converting trucks to LP gas you might get a deal there on a tank removed from a big truck. I have one awaiting it's rebirth that's around 23" in diameter and 6" long that's about 3/8" thick that came off a retired concrete mixer truck. I haven't yet decided whether it's going to be a trailer mounted smoker or get whacked in half to make a pair of patio size smokers with warming boxes.