Even when a 42 gallon tank has the correct amount of air in the top, it still only holds about 10 gallons of usable water. The less air in the tank the less water it holds and the more the pump cycles. The 20 gallon tank only holds half as much and cycles for every 5 gallons used. There is really no problem with a diaphragm style tank out in the open, as long as it doesn't freeze. The old galvanized tanks usually have a bleed back system that lets the pipe to the well drain back every time the pump shuts off. This keeps the line from freezing, and puts a little shot of air in the tank each time the pump starts.
If there is a "bleeder" or drain back valve down 5' or so in the well, it will need to be removed and plugged before switching to a diaphragm style tank. The line will no longer drain back, so it could freeze, and it will not send a shot of air to the tank each time the pump starts. With a diaphragm style tank the air is captive above the diaphragm. So you do not want a shot of air from an air maker type system, or it will just blow a glass out of your hand at the faucet. Removing or finding out if you even have a bleeder in the well is usually the hardest part about switching from a galvanized tank to a diaphragm style tank.
Now once you have done away with the bleeder system, switching to a diaphragm style tank with a CSV is easy. With floats in water tanks like that I would use one of the PK1A kits with the 10 gallon size tank. Float systems are notorious for using water for long periods of time at really low flow rates. This is exactly what a CSV does, keeping the pump from cycling while using low flow rates. But if it is using less than 1 GPM, the 10 gallon tank is preferred over the 4.5 gallon, as it will lengthen the cycles considerably.
