I have two portable air tanks that have factory installed fittings that hold a safety valve and the gauge.... I would favor leaving this part as is,,, and add a quarter turn valve and a new 6' line with a coupler. to fill the tank, a simple adapter made of two male ends coupled together..... Safe, efficient, and you can use the air chucks and blow guns of your choice.
The advantage to the valve on the tank, is you can use a blowgun that does not have it's own valve..
simpler yet... just put a 6' hose on the tank, with a coupler on the end.
to fill it, a quarter turn valve with two male ends ,,,, turn the valve off, couple into the shop air, and couple on the hose from the portable air tank...open the valve... the worse that can hopefully happen is the safety valve on the portable air tank will release...at that point, turn your quarter turn valve and unhook your couplers.........pppppppsssssssss you can use this one simple adapter to fill any air tank that has a coupler on the end of it's hose...