What is the trick to getting the valve cover off a /6 with AC?

Schneiderman said
It's a 1970 duster.

For next time: You needn't remove the "AC unit" (think you probably mean the compressor). All you have to do is very(!) slightly loosen the nut at the firewall end of the hose, then pivot the hose upward and sneak the valve cover out from under it. Once the valve cover's back on the head, hold the hose so the firewall end fitting is horizontal, then retighten the nut. Thereafter, all you have to do is flex the hose up out of the way with one hand while you remove the valve cover with the other. Not difficult.

See the valve adjustment procedure.