I was reading the super-six document, and ran across a warning that a bog may be caused by a blown vacuum advance pod. I've hooked up a hand vacuum and it holds a vacuum. But I suspect the insides may be seizing up due to rust or something. The timing gun shows it is advancing...sometimes. Can this part be replaced without replacing the whole distributor? Is this a common problem with an easy fix?

Your distributor's advance mechanism is gummed, need to disassemble and clean old hardened grease off the parts and lubed lightly with proper lube (don't know which lube).

Cheers, Wizard

Changing the VA "pod" is easy, 2 screws hold it to the side of the distributor. You will have to remove and reinstall the distributor to do the work so rotate the engine so the rotor is pointed straight up (12:00 "high") prior to removing and then re-install in the same position.
DD