I'm guessing your distributor is stamped 1111270 which calls for a
1973412 vacuum control. The manual trans version (1973411) has a 20 degree advance and 10 degree retard as in this NOS one for sale on Ebay:
http://www.ebay.com/itm/314412325845...OPIC%26ao%3D1%
Another (used) 1968 vacuum control on Ebay (for big Pontiacs) is also 20 degree advance which would be a good bet that all 1968 Pontiacs with the dual port vc requires 20 degree advance?
If so, then your "other equipment" dual port vc should work fine if stamped 20A...just disconnect the retard port.