You probably hit the nail on the head with the mechanics not know how to do it. All they are used to now are cars where they can "set the toe and let it go" and most still don't get that right.
Alignment shops usually rely on some sort of computerized system based off of factory specs. The way it works is you set the car up on the rack with turn plates and sensors. Then you input year, make, model etc. and the specs come up on a screen. Adjust the components to proper range and test drive.
Keep in mind these specs are based on the assumption that the car has all the original equipment and tires and is set to factory ride height. And, though I am not 100% sure on this, most shops probably don't have accurate information in their alignment system for anything before the '80s. They likely won't do it because they don't want the liability of a 'custom' or guesstimate setting which may differ from whatever info they can find.
Could they do it the way you want? Probably but again, no one wants the liability.
