I was in the same boat rrenz. I was able to put together the correct vacuum advance by buying the parts. I was not able to acquire a complete restored unit so I sourced the parts and put one together. Maybe someone on the site knows where somebody that restores vacuum advance units?
You can get a correct DD-190, C5AZ-12370-A from Tim o'Conner. His website says he closed but send him an email (see his contact page) and ask if you can still buy parts from his website. This was the same situation I encountered but found he was still selling parts from his website. Ask him if he has the threaded end cap for the vacuum advance and the internal springs and washers. He had a new/restored end cap when I asked so I bought one.
http://www.428cobrajetcars.com/new_partsIf Tim does not have the end cap, internal springs and washers, see if you can pick up an old correct vacuum advance or distributor with the correct vacuum advance intact on eBay. IIRC, I was able to pick up a gutted distributor with a bad vacuum advance (the diaphragm was shot) but it had all the internals for about $30. The springs and washers are used to set the advance characteristics. Here's some pics of the internal springs and washers (end cap on right). The top parts in the picture are from a 64.5 260 Auto, the bottom are from a 65 289 Manual.
Lastly, be sure to get your distributor re-curved. Dan Nolan at Mustang Barn can do this for about $85.
http://www.mustangbarn.com/Good luck.
Mark
