Acetylene tanks cannot be used for any other gas. They are filled with acetone; the acetylene is actually dissolved in it. The tanks operate under a relatively low pressure -- a few hundred PSI.

Oxygen tanks would be mechanically suitable for argon, having a pressure rating of 1800 to 2200 PSI. But they have special fittings and are painted a special color. The supplier will not fill them with anything but oxygen.

You will have to obtain a true argon tank.