Hmmm, well I have a different viewpoint on this one. There certainly are times when manifold vacuum would be a good idea (.750" lift roller cams come to mind), but Mopar designed the "stock" engines to run with venturi/ported vacuum. How is your motor built? It's true that when you floor the pedal the vacuum advance stops almost immediately & almost completely (vacuum at WOT is maybe 2" in. Hg or so). The mechanical advance will help....eventually.... as the rpm's increase. If you floor it from idle, you'll notice.
Regardless, when setting the initial & total timing, the vacuum advance on the distributor should be blocked off while you're checking/setting the timing. I agree with Bill that you should set your initial timing (and idle fuel mixture for that matter) by maximizing the vacuum and rpm's at idle. That's how you know you hit the "sweet spot" in initial timing that your engine likes. Next, you rev the engine up slowly, while watching your timing marks, to see what your total advance is. 36-degrees +/- 2-degrees is typical total advance. Also note at what actual rpm you're "all in".
Now the fun stuff..... IF your total advance is more than 36-degrees, then you'll likely need to limit the total advance. You can do this with little advance limiter plates, built-in adjustment screws on some distributors or the old "weld up the advance slots & then file them away until you hit the limit you want" technique.
Then the good part.....now, if you like, you can take a look at your advance curve. This is basically swapping out the two mechanical advance springs stronger/weaker or adding/subtracting weight. That allows you to custom tailor exactly when & how much mechanical advance comes "all in".
Lastly, re-connect the vacuum advance (if you like) and see how the car drives.