Proper Way To Cut Copper Oil Line

[1972BlueC20]

I need to cut my copper oil line under the dash so I can install another fitting on the line to temporarily hook up another pressure gauge to verify my oil pressure.

What is the correct way to cut the copper oil line without pinching it off which would restrict the flow.

I tried just pulling off the little ferrule crush sleeve thing in front on the current fitting so I could slide the fitting off and then put it back to the old fitting later but I can't get it to budge even when tugging on it lightly with pliers.

My aftermarket temporary oil gauge requires another fitting than what it used to hook up to an original cluster oil gauge. I tried to find an adapter to convert the current fitting to what I need but can't find one.

Any help is appreciated