Nice stand!
Plunger is a fuel pump, like the bulb in a modern single line system. If the motor isn't ready for prime time yet, you could disconnect the lines from the motor (probably want to replace them with fresh lines anyway). Connect the tank as you normally would to run. Put a little gas in the tank and use the plunger to pump gas into a container -- tests the pump, but also cleans out the tank and hose. On the air side, you could rig a hand pump to get some air into the tank, then crack the cap a little like Racerone says (should hear a hiss) -- or pinch the line, put on a pressure gauge and see if it holds. Based on some info F_R provided in an earlier thread, there is not a lot of pressure supplied in normal operation (maybe 5 lbs), so don't overdo it. I don't think a MiTvac would work to test tank integrity because a check valve would interfere. (Maybe if it held a vacuum that way, you would know the tank was not leaking -- except through a bad check valve!)
Anyway, that might be a way of testing, prepping and generally tinkering with the tank before the motor is ready. I have an old motor connector end that I occasionally use to pump out tanks/lines.
Think you have a slightly earlier tank there. '57/'58 model would have a plastic plunger cap, I think. They work the same, and only some twit with too much time on his hands would mention a difference. If detailing the tank, I would just get the appropriate decals (script design for '57 and probably '58), and go from there.
