You might get away with just checking the stock '66 distributor and make sure that it is in fact providing enough advance at idle and when the throttle is openned up. I had my 2 bbl carb hooked up to the stock distributor for a year or so, before I found this site. I had been unable to get the car timed per factory specs and tuned it by ear. Turns out I was dialing in a bunch of initial advance to compensate for a weak vacuum diaphram on my distributor. I am not exactly sure how the spark control valve on the stock carb regulates the vac signal to the stock distributor, but seeing as how the stock distributor only had vacuum advance, I did not see how you could get enough advance under wide open throttle using an aftermarket carb. Under wide open throttle, engine vacuum drops when you punch the gas, and the ignition timing would also be reduced resulting in slower acceleration. Some of this reduced timing is required to prevent the car from pinging. The centrifugal advance component of the timing will help "bridge the gap" better when vacuum is reduced and timing is reduced under wide open throttle condition. As the car speeds up, the engine vacuum increases the vacuum advance, and the centrifugal advance also increases the timing resulting in more power. The parts store did not list any replacement vacuum cans for the 6 cyl cars so I replaced the stock distributor with a later model that has both centrifugal and vacuum advance. The distributor cost about $35 from Autozone, and then a new Pertronix Unit was about $80 or so after shipping. The 66 and the 68 distributors take a different Pertronix unit. An alternative would be a Duraspark II distributor and module. I think the prices for a rebuilt Duraspark and ignition module would be cheaper. You can find further distributor info by using the search feature.
. 
