As I remember it there was a trick some guys used to do with the old metal tanks (I believe the 97 up were plastic , might be wrong) to get more than 20 gallons in them. I believe the tanks them selves were bigger than 20, but the way the baffles of something were arranged to allow a certain amount of air even when full kept capacity at 20. It was something fairly simple but I can't remember what it was.
I doubt the new tank is different in its dimensions, just because all the existing hardware and hangers would need to be swapped as well, but not impossible. I'll try to find the old tank hack when I get to a pc