There are no "smog add-ons" with regard to the large cap distributor. It has two vacuum ports, the inner one is for the typical vacuum advance mechanism sourcing vacuum from the carb base, and the outer one for the high altitude compensation circuit. You may choose not to use the HAC or delete it as part of your de-smog (I did). Removing the HAC just means you won't need to use the outer diaphram port on the distributor.
What is "smog" about the big cap distributor is the built-in advance curve. The distributor uses a system of weights and springs inside that, when spinning with rpm, advances the spark timing. The weights and springs Mr. T put in that unit were chosen based on smog equipment on the engine being intact. Once the smog stuff is removed from the engine, the distributor will advance timing too quickly and too far for a desmogged engine, causing detonation (pinging). Hence the need for a recurve. The recurve generally consists of changing the springs and /or weights to slow down the vacuum advance. Some folks choose not to recurve and instead use only the outer HAC port for their advance. This works okay because the outer port only allows 2-3 degrees of advance. Other folks will use the inner port, but decrease the initial timing of their engine, so that the pinging goes away. I am a fan of recurving the distributor as opposed to doing one of the work-arounds. I've tried them all, I just think the truck performs best with a recurve.
