72 A/C Low Side Valve

I take it you are using the old V-2 cast iron compressor. You can leave the old fitting in the head. Then your options are to either install a new valve in that fitting and install a hollow adapter, or remove the valve in the fitting and install an adapter with a valve in the adapter. The valve in the fitting in the head are similar to a tire valve, and are removed and installed with the same tool. But do not install a tire valve, it will leak eventually.