Only 16 inches of vacuum, that might be a clue, possible exhaust dilution at idle. Bad gasket at EGR valve or a bad seat in the EGR valve itself may not be ceiling. Turn the EGR valve up and pour water in to the seat see if it holds. Check gasket surfaces for flatness.

Bad gasket where that red arrows pointing, and if gasket is still good around the green line. Spraying carb cleaner from the outside may not detect this leak. This is a somewhat rare failure but I've had both the gasket fail and valve seat leak. Probably not your main problem but it is worth checking.

EGR was removed by a prior owner. There is a blockoff plate installed and I replaced that gasket when I got the truck five years ago. I will double check its integrity.

so the original carb is likely still allright?

Probably. That carb ran fine for years and just suddenly stopped idling.

Last night I replaed the coil with an old MSD Blaster II coil and I replaced the cap, rotor, and wires. That seemed to help, but there are enough other electrical gremlins cropping up that I am afraid the truck is very close to not being a reliable daily driver. Time to park it and do a full resto, or figure out what to do.

I suspect you still have a vacuum leak in the adapter stack.

Is this what your stack or adapter looks like?

A big restrictor plate....

I will get some pictures. Give me a bit.

My adapter and gasket stack goes, from bottom to top,

1- intake
2- thick BBD gasket
3- phenolic heat shield specific to the BBD bolt pattern
4- gasket with the elongated bolt holes for the small and large 2bbl bolt patterns, but WITHOUT the holes for the small four barrel mounting pattern
5- two to four-barrel carb adapter with the two-barrel side down
6- four barrel squarebore carb gasket

Complete stack