The dished in end will blow out the end if it fails should not be too much shrapnel.

The end that goes in is dangerous. I would suggest that you keep the pressure below 60 psi. Even if you test a tank like that at high pressure and it holds it is no guarantee. Every time you fill it the dished end presses outward on the tank and stresses the welds. The dished ends of old water tanks...

Is the end of the water tank dished inward or are both ends domed outward?

A welding supply store wil havel an adapter to mount your argon regulator on the CO2 tank. You could also get a new inlet fitting for a CO2 regulator and put it on an oxygen regulator, the inlet of both should be 1/4" npt.