Hi Keengardener and welcome to GoY. I think it very doubtful you will be able to move a 50 year old grape vine and have it survive the move as the root structure will go well down into the ground. As you have to move you need to do so as soon as possible as the sap will already be rising. Carefully dig up the vine with as much of the rootball as possible, prepare a hole the same size as the rootball, plant and water in. You can't even prune it right now as the vine will bleed sap for days.

Best idea is to build the extension over the Grapevine and retrain it round the walls ! A productive Vine is well worth keeping.