I just did one from that vintage style.

What I found was the tumblers were so full of crap that the key could not pull the wafers down into place. I took each one apart and reworked them, but a good attempt first will be some BP Blaster into the key slots and then some air to blow them out. [ let the BP sit for a while]
The main problem with these old tumblers is they get worn from key insertion to the point that they will not lift the wafer/tumblers far enough for the cylinder to get past the shear line. A simple fix w/o rekeying is to take the cylinders out and insert the key..then file down the offending tumblers that are too high until they are even with the shear surface of the cylinder it takes very little filing andit is just to make up for the wear amount on the tumbler..an emery finger nail board works good ..Then when you put the key back into the cylinder , all the tumblers are even and the key can then be turned. The tumblers slide into a slot and the key will not turn until those tumblers recede into the cylinder ..only then can the key be turned..........
It was easier than one would suspect and beats the Lock Smith trip.
An easy one would be to start with the trunk one to see how they work..that one is just held on with a snap ring and then the cylinder comes right out..you will then see how they work. Or the glove box one.
Try the BP first.........

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A Dalton