There seems to be some confusion here. Running with, or without vac advance is not critical, either way. My position is simply that if you eliminate the vac advance system you will be able to calculate, in advance, what your all-in timing will be under any driving condition. Up hill, down hill, cruise, WOT accelleration. Also, based on test drives after any modification, you will be able to reset without any doubt as to what the timing is doing.
The goals. First, the car needs enough initial to idle nicely, in gear (stick guys won't appreciate that one). I think we can all agree that this number is in the area of 15* (+/-) with no input from the mechanical or vac systems, i.e., the purpose of slowing the idle down to 500 / 550 and disconnecting the vac lines is to eliminate these elements when setting the initial. Second, there is much evidence to indicate that the all-in advance, the advance needed by these cars over 2500 RPM, is in the area of 35*.
Now, having said that, we all know how to adjust the initial. As for the all-in, the mechanical advance limit in the dual advance dizzy can be adjusted by changing the weight limit in the dizzy, as shown at the web site above. If we assume that we want 15* initial, and 35* all-in, then we must have a 10* weight limit in the dizzy, which will give you 20* at the crank (dizzy advance is 1/2 crank advance - the dizzy turns 1/2 to crank speed). Bottom line, if the initial plus the mechanical will give you the 35* target number, you don't need the vac system. The vac will simply overadvance you at idle, causing you to need to back off on your initial after a test drive. Then, under WOT conditions you may, or may not, get enough advance with you new initial setting. At issue will be how much vac the engine pulling at that specific moment and what does that translate into vis-a-vis advance. Personally, I don't think you will ever know. All I can say is that, according to Ford, the vac will overadvance you at idle, and under a "no load" driving conditions, i.e., when cylinder pressure is low. Their purpose for this technology? Reduce NOX and increase mileage. In my opinion their position has not been verified. The headache of dealing with this now very old technology, on the otherhand, has been verified by owner after owner who has reported timing / idle issues.
Summary, if you want to know what your timing condition will be under any driving condition, if you want to be able to adjust for mods easily, try the out. It's not hard, it's not epensive (if you need to buy a dizzy ask for one with the 10* limit - save some time and trouble), and you will like the results. However, it will be necessary the test drive and adjust the initial up, or down, to get the best results, i.e., the highest initial advance you can set without ping during periods of high cylinder pressure (WOT accelleration - hill climbing).
Enjoy the trip - Steve