Howdy PR66:
To clarify, the DSII came in two basic versions. Both have centrifugal or mechanical advance and vacuum advance. But some came with a single vacuum advance cannister, identified by one vacuum nipple on the end of the cannister, and a dual vacuum advance/retard cannister, identified by two nipples- the second, being between the cannister and the distributor body. Either will work better then your stock '66 vacuum advance only distributor, but the dual vacuum advance/retard system is a little more complex and troublesome. To get it to work as it came from the factory, it requires at least one vacuum switch that allows the system to change from advance to retard, depending on several factors.
Both types of vacuum advance systems were designed to use a ported vacuum source on the carb. I believe the Holley #1940 is a replacement carb, and should have a ported vacuum source located above the venturi.
The big differences between your original ignition system and a DSII are the addition of electronic internals and centrifugal advance.
The DSII ignition system, including the distributor, coil, module and wires, should be a nice upgrade for your engine. Set the initial at about 12 -15 degrees and regap your plugs at about .045" for old, and .050" for new plugs. You will likly notice quicker starting, a smoother idle, better mileage and acceleration and more top end.
Enjoy!
Adios, David
