I have run into a problem with getting a 1970 Challenger [383 4 BBL] with a 4369 Holley Carburetor to idle properly. It maybe that in order to get the carburetor set properly, I will need to know what the factory vacuum specification is. Does any one know what the factory vacuum level is for a that set up?
This is the original carburetor, but I may need to consider another option. Any suggestions?

Classic car insurance?

Timing using the factory specs?

If yes to both, throw the factory specs in the garbage. Time it with about 14* idle timing, fix the mechanical advance slots so it doesn't ping.

What is your initial? Is everything (esp the cam) stock? and how mild/wild a com is it? Are you on ported and what is the lowest hot idle speed it will idle at (& is it a stick or auto? & if auto is that in gear/foot on the brake (or ebrake on).

Iron head 383, 9.6cr, very close to stock cam, exhaust manifolds= Steady 19" manifold vacuum. I've heard people claim as high as 24" but that seems too high.

that is real good vacuum. what is the initial/are you on ported/lowest idle speed you can get/is that in drive or in neutral

Initial 16* 34* all in at 2800, vacuum plugged, with vacuum attached I'm set at 30* drive idle 750rpm. Neutral 850

Recently installed an FBO distributor advance limiting plate.

I know Don @ FBO is a manifold vac afficionado. Not me, tho I have heard good regarding it from some people & I admit I just ain't never worked with it to honestly evaluate it. Your numbers all sound good except I could see the 30 initial on manifold giving some idleing problems. What I would do is go to ported & set the initial at 16-18 (& confirm you ain't into manifold vac at your idle speed & it sounds like you are already good on that) then set the plate to 36 or 38 total (initial+plate) then plug in the can

The 1970 Challenger is an automatic.

The timing has been set at about 10 degrees at present.

Currently, the vacuum is ported.

The cam in the vehicle is certainly a more aggressive cam than the factory cam.

This is the current belief. I suspect the power valve is opening early [i.e. near idle], and dumping excessive fuel into the carb [running rich]. On acceleration, the belief is vacuum drops off. I am not sure what the rating of the power valve is, but it could be between 2.5 inches to 7.7 inches vacuum. So, if I can understand what the factory vacuum setting for vacuum are, I can change the power value to compensate.

Hence the question, what is the factory engine vacuum for a stock 383 4 BBL?

I cant say that there IS a vac spec. I've heard that the PV should be half of your idle vacuum (from Holley iirc) and I've heard that you want the PV several in hg numbers below what your idle vac is (to me that seems like you'd be in the power circuit almost all the time). First we need to find out if it is blown: if it is idling low enough that you ain't in the transition slots then you can turn the mini idle mixture screws on the side of the metering block in CW & if it dont die then the PV is indeed blown. I would (1) find out if the current one is blown (2) get the initial finalized (amt/ported or non ported (3) note the vac at that point (at your hot in Drive idle speed which is what you will be at stoplight to stoplight) (3) If the PV is bad (or good) decide what PV # you want & if good if you want to change it. (& maybe someone has a # for you per your request). EDIT you definitely need more than 10 initial right now no matter what (I had 18 on my bone stock '83 318 DD)

If you have a larger cam than stock, increase initial timing. 10 is stock 318 territory. Maybe double it and see how the idle does. See if it starts hot without kickback or serious drag on starter.

The PV should have no effect on idle if the ignition/carb are set up properly.