to me the air brakes are easier to work on. That vintage with he dayton style wheels you proably will have to pull the whole wheel hub assembly to replace the shoes but the adjustment is easy usually just a 9/16 wrench needed. Dayton wheels means an easier time finding suitable replacements if any are in rough shape
Is it a tandem set up with 1 axle live and the other a tag axle? or is it a full twin screw? I'm not too familar with gas tandems of that era. As far as it's value it's whatever you feel comfortable with.
The transmission may free up when oil has been circulated a little. it's hard too get an idea with out the truck running
