Fine Tuning a 360 Magnum with a Brawler "Double Pumper"

that cam is 264/270/114; 212/218@050
The comp timing specs are impossible for me to get sense of, so I'm gonna assume that you installed it at about plus 3*, which is in at 111*
If that is true then your cam-timing events are;
264Int/117Comp/108Power/270Exh/39overlap/Ica of 63*
If you are getting 20" of idle vacuum, I can almost guarantee that the throttle is too far closed.

You cannot run like this in Virginia, at this time of year, especially at altitude, without a heated air intake.
What happens is that the edges of the throttles will form ice, and the engine will slowly choke. When you blip the throttle, some of that perhaps all, will break off, and disappear into the engine. Then it will idle fine for a few seconds and then it begins to choke again.

Reset your Idle-timing to 10*, increase your idle-rpm to 650 in gear, twiddle the mixture screws to best rich idle. Make sure the Secondaries are fully closed but not sticking. Make sure your PCV is plumbed to underneath the Primary throttles, which will help warm the incoming air, and finally, install a heated air system, that draws hot air directly off the exhaust manifold, into the airbox; exactly as the factory did.
IMO, you are attacking the rich-running from the wrong angle. see note-1
re-Install the original MJs of 66/71; leave the 6.5PV alone for now. Verify that your WET fuel-level is where the designer wants it to be, and that it STAYS there.
Now, fix your stall timing, see note-2
After that see the other notes.

Note-1
>Your low-rpm low-throttle-opening fuel-delivery system is;
the Transfers, plus a lil trim from the mixture screws. For the most part, this system is not adjustable.
> the PMJ comes on line at about 20>25 mph, when transitioning from steady-state cruise, to lightly accelerating. You can dick with the PMJs all you want, but at steady state, at 20 or so mph, they are NOT YET flowing.

Note-2
Your engine has three Ignition Timing Milestones. In order of importance, they are;
Power-Timing; usually about 34>36 degrees
Stall-Timing ; about 1 degree ; per 100rpm. If you have a M/T, consider this to be in the window of 2400>2800.
Cruise-Timing; depending on rpm can be 50>60 degrees
Idle-Timing ; not real important, except;
a) a tip-in sag sux. If yours does that, the throttle needs to be opened further.
b) banging into gear also sux. if yours does that, then retard the Idle timing. If you have a M/T, ignore this.
c) other than that, target 10>14, I say target.
Your job, as a tuner, is to marry those 4 points by modifying the distributor.
Power-timing of say 34*
If you convertor stalls at say 2400, Or if you have a M/T, then your target is 24*@2400.
If you need to run ~10* Idle-timing, then the D will have to add 34 less 10=24 degrees, install the appropriate cam.
After that, you need to connect 24@2400 to 34@3400ish, with rate of advance, which in this case is, ~1* per 100rpm.
Then connect 10* at Idle to 24* @2400 which is about
14* per 1600, which distills down to .88* per 100.
This is so close to 1* per 100 that you can just install whatever springs get you to ~1*per 100, then set your Power-Timing to 34>35, and let the Idle-timing be what it will be, she really doesn't care. Then roadtest, listening for detonation and modifying as may be necessary.

note-3
for winter, I would run the smallest radiator fan, on a thermostatic clutch, that gets the job done. This to reduce the draft over the carb/intake. and I would run a heated intake. With an electric assist choke. But if the AG is retained, then a manual choke.

note-4
After the Ignition-timing is fixed, now you can finalize your carb adjustments.

Note-5
lets talk about that cam.
Because of the very wide LSA, the compression degrees are just 117, and the power degrees are just 108.
That 117*, makes the Ica to be 63*
The Wallace Calculator predicts a cylinder pressure of just 144psi at 9.2Scr@900 ft elevation. And that means, that your 92 Dakota, may be be a lil sluggish off the line, depending on gears (and stall). Don't try to make it more than it is.
However, it also means, that the throttle is gonna need to be opened further during acceleration, all the time. and that is gonna suck gas.
Furthermore,
Assuming no headers, the Power stroke is just 108*, so when the Exhaust valve opens, there is still a good amount of pressure in the exhaust, which is pressure that should have/could have gone to the driveshaft; which sux gas.
Furthermore, your pistons have to pump those higher-pressure gasses, out of the chambers which, you guessed it, sux power and thus gas.
Therefore, for fuel-economy, your truck should have a high-efficiency dual exhaust on it.
But if you have headers, it might be worse, as the headers suck on the intake valves at the end of the exhaust stroke, which at low rpm, can suck thru the overlap cycle, all the way to the plenum and pull raw A/F mixture straight across the piston which is parked at TDC, and out the header it goes. Now, your cam, has a very modest overlap of just 39 degrees, but remember the factory 340 cars? Well they had just 5* more and were known to be gashogs. And guess what else; they had about the same Ica and just 104 degrees of PowerStroke.
The point is that you cannot expect that low-pressure combo to make fuel mileage. You need a different cam and/or more pressure.
And all the playing around with the carb, is only gonna get you drivability issues. Fix the cold-running issues, and fix the ignition-timing and engine-wise, that's as good as it gets.
At some point, I would try to take the Primaries down a jet or two (probably no less that 68), and kick up the PV to 10.5 which makes for a smoother roll on with a too-lean PMJ.

Note-6
These are my opinions; (both above and below)
As for me;
1) I would never install a 114LSA cam into an SBM, no matter what. All your fuel-economy problems could have been avoided with same cam ground on a 106>108 LSA.
2) Notta chance would I ever run cylinder pressure as low as 144psi, in anything but my winter beater.
3) Notta chance would I run less than a 2800 flash-stall.
4) I gotta have headers.
5) Notta chance would I run a rear gear less than the equivalent of 3.55s with 27/28 " tires.
6) if I had an overdrive, I would gear your combo to cruise at no less than 65= 2200
7) I would crank the cruise-timing up to 50>56 degrees, and see how she likes it.
8) I would try to run PT-timing a lil more than 1* per 100, from Idle to stall, and to prevent detonation; either
a) slow the rise after that, or
b) delay the all in to a higher rpm, as may be necessary
9) the 92 Dakota is a brick; I had one too, allbeit a lwb/extended cab/4x4/auto, and it sucked gas even as an MPFI/ A518/ 3.91s. Just make sure the brakes are not dragging, the alignment is good, and the tires are hard; what else can you do......
10) BTW, you said double pumper. I would Disconnect the secondary link and wire the secondaries shut; you may be surprised at how soon the Secondaries begin to come in.
On my hotrod, I designed an over-rideable primary throttle-stop, which I adjust to get me just enough throttle to cruise with. Thus I always know how far the Primaries are open. and Yes, when on the stop, it takes a really long time to come up to speedo
Ok so, I'm going bak to bed now, lol.
Happy HotRodding