shouldn't be a problem. I don't know the details of the 60s stringers, but the 70s models are easy. Just put the A/C in place of a vent--the vintage vints aren't "standard size", which is 14"x14", but it's relatively easy to cut the inner and outer skins to the larger size. Your problem will be getting 115V up to the opening and deciding how you want to handle the condensed water. Stock Airstream A/C installations have a tube running inside the shell. Newer A/C units (at least the one I have) are designed to flow the water outside the shell (leaves a stain, however, which is a bummer).
Take a look at this thread and you can see how I did mine in the Caravel
It's very important that you strengthen the area around the vent opening. Installing the wood blocking (per the photos in the above thread) results in remarkable structural rigidity--you can easily do a pullup hanging on the edge, so the reinforced shell can readily support and A/C unit.
BTW, the newer A/C units have their themostat built into the unit, so you don't need to run thermostat wires anywhere.
Please post photos and details of how you find the 60s shell construction. And how you get the 115V up to the A/C.