70 Duster runs rough or dies when up to operating temps

I'm with Bewy, post #5 and offer this;

Your 150 rpm drop may be caused by; Not enough throttle-valve opening, caused by too much timing.
The proof is in the mixture screws being out too far and black sparkplugs.
The stalling could be because of sooted up plugs, or a too high or unstable fuel level.
The 15" of vacuum could be a bit of a cam, but without hearing it, it just as easily be a stock cam and a vacuum leak, or more likely a flooding condition..

I would begin by setting the Transferslot exposure under the closed throttle to a bit taller than wide, and closing the mixture screws to 3.0 turns. After this, do not readjust the speed screw!
Then reset the idlespeed using timing, to 550/600 in gear. I'm assuming you have a good working PCV system, no vacuum leaks and the coolant temp is around 185. Do not pay attention to the actual timing number just now, just make it idle as above.

A flooding condition is easy to diagnose, now; just pinch the fuel line right closed while the engine is nearing a stall, trans in Park. If the rpm starts to recover, there you go, the fuel is going too rich. Let it idle until the rpm peaks. then shut it off. Pull the top off. Whatever the fuel level is in there now, record it. It will be a lil lean for day to day driving but it will be close. And it will be your target WET fuel level. So make a float adjustment towards a lower level and repeat the test. When you get it synced up, time for another road test.
But first; reset your idle-speed to 550/600 in gear by using timing. And then fiddle the mixture screws for smoothest idle , between 2.5 to 3.5 turns. If it wants more than 3.5 turns, she is lean so give her a half a turn more on the SPEED SCREW, and twiddle the mixture screws again between 2.5 to 3.5 turns. If she wants less than 2.5 turns, she is fat so subtract transfer fuel by decreasing the speed screw by .5 turn. Depending on how many changes you made, the idle speed should not have changed much, but if it did, well, put it back to 550/600 in gear.
Now it's roadtest time.
Pay attention to freedom from; a tip-in hesitation, to response to gentle accelerations, to the rpm or speed they occur at, cuz we will have to fix that later. NO WOT bursts yet. Then take it home and see if it stalls again.......gotta fix this first. If it does it again, my money is on a leaky float valve, so you'll have to pop the top and compare the current WET level to your previously recorded numbers. If they differ by much, you have a choice to make, but first go look for debris in the valves. If none found, the choice is to reduce the fuel-pressure or to replace the valves. If the pressure is already below 4.5psi, just buy new valves. But before you do, if you have brass floats, make sure they do not contain fuel; take'em out and shake them. If they do, you have another choice; fix them or replace them.

Happy HotRodding