Having a brain cramp! Holley 750 DP - is the distributor vacuum advance off the carb base manifold vacuum or is it off the ported port?

base is manifold. side nipple on metering block is ported (what works best for our Mopars)

There's two schools of thought on this. If you want better off-idle-and-up drivability, performance and mileage use manifold vacuum. If you want no vacuum advance most of the time except at high speed on the freeway or at medium-to-high rpm with less NO2 emissions use ported.

Either way, you need to have your distributor set up right.

ported; at the metering block.

Typically you want ported vacuum so you don't have advance kicking in at idle. With the ported vacuum you don't get a vacuum signal until the throttle blades open up a small amount. Once the throttle is open a small amount ported vacuum = manifold vacuum.

I used to think it made more sense to use manifold vacuum because it will give you a better idle. However, I've changed my thinking. I like to use ported now. Get the base timing you want in your timing curve. Otherwise when you stomp on the throttle from idle you're retarding your timing and losing response.

However, if you don't know how or don't want to recurve the dizzy, manifold vacuum will help give you a better idle easily.

You can really set it up however you want though if your customizing it. Older cars used manifold vacuum, most of our cars used ported (anything mid 60s or later). As Andy said above its the same, the ported vacuum is just not active at idle.

On a car with a big cam if the vacuum fluctuates a little at idle running manifold vacuum could cause fluctuating idle timing and make the idle RPM jump around alot. Of course, the idle vacuum fluctuation is exacerbated when it starts pulling and adding timing too.

Manifold vacuum has always worked better for me, more timing at idle, helps the engine idle cooler, and you can turn down the idle screws on your carburetor because you have more advance at idle to help the engine idle.
Every engine I've ever been around idled better and cleaner with the vacuum advance on manifold vacuum

an option would be to get the FBO plate ($25 to your door) or make your own & dial it in in both ways & see which one works the best for you. (4) subsystems: initial/total/spring rate/vac can

If you have a distributor setup for ported, then use ported. If you have a distributor setup for manifold, then use manifold vacuum.
Dont try and run them opposite and you will be fine.

^^x2.
My choice has always been and will always be ported. When ported can't work any more then plug it (rare occasions, and never a street car). That being said it's simply not worth arguing about.

I cant say I have ever ran manifold mainly cuz I am used to em being plumbed that way OE (ported) & I've only had one carb I've dealt with that had no ported nipple so I just used manifold automatically (had no choice there) & didn't really dial it in for spot on) but sometime when I get caught up & I'm gonna try it (& dial it in for manifold) just to see/be informed & I may like/prefer it

Most manufactors used ported vacum. Basically as Andy F said ported is manifold vacum but you just dont have it at idle because the hole is above the throttle blade. Once you hit the gas and the ported hole opens its just manifold vacum then which will be strong and a part throttle cruise and even at about half throttle. Myself I feel ported works better as I like it to bring the vacum advance in after you hit the gas pedal.
Ford actually used a thermal valve on emission engines screwed into the water jacket that if the eng started to overheat at idle the valve pushed a ball up to cover the ported hole and send full manifold vacum to the dist to speed up the eng some and pull more air through the radiator to help cool it down along with the advancing of the timing. But you can set an eng up with enough timing at idle to run good and not run hot without the vacum advance coming in at idle.
If you use manifold vacum at idle when you step down on the gas a little you wont get anymore vacum advance but if you set the right timing at idle without the vacum advance then you get that extra timing coming in when you step on the gas to help with power and help to not have any hesitation.
I know most all the cars I worked on used ported vacum on them. Ron