Sixmaniac:
Your truck has "Transmission Controlled Spark"; an early emissions control gizmo that came out on the '72 GM trucks.
My '72 Chevy originally had it, along with the Rochester MV carb. Then some previous owner removed that carb and substituted an earlier Rochester B carb.
I have yet to get the vacuum advance to work correctly- it presently works "backwards": full advance at idle, and less & less advance as the throttle is opened.
Now, this is how the shop manual says the systems is supposed to work:
The system consists of a switch located on the tranny, a relay on the cowl, and a solenoid on the carb that also has a couple of vacuum nipples.
The vacuum hose goes from the carb base to the solenoid, then to the distributor.
The idea is that the distributor vacuum advance gets vacuum only when the engine is cold and in first or second gear.
When running in "high gear", the vacuum advance is "locked-out", and the solenoid is "energized", which causes that little plunger to move out slightly, and prevent the throttle from closing completely, thus (hopefully) preventing over-rich mixture on deceleration.
This is somewhat paraprased; the big point is, if it's been altered, you'll have a lot of trouble getting the engine timed.
My best suggestion is to get a factory shop manual for your truck: it has a pretty clear description of the TCS system and how to work on it. ($30.00 from Jim Carter or one of the other suppliers- IT'S WORTH IT !).
When the Spring breaks, I'm going to scrounge up the missing parts to put the TCS back on my truck, and save the Rochester "B" for an earlier vehicle.
