My idel is low enough (about 500 RPM) when the engine warms
up that with the weak battery now in it I see low voltage on
the voltmeter.

Sometimes, when I goose the throttle (link when maneuvering in
tight quarters) it stalls too.

I have the E4OD tranny, but it sometimes even will stall with
the engine in park if I rev & release the throttle.

Is there a way to adjust the idle? I see a threaded stop screw
of sorts, but it has no screw head or other way to turn it (short
od grabbing it with pliers and mangling the threads...), so I assume
it is not meant to be a throttle stop-screw.

I read something about an IAC valve in a 5.0L message thread, but
I didn't find anything for the EFI 7.5L idle adjust (I tried... that doesn't
mean I didn't miss it).

Anyway, my question is two-fold...
Should the alternator be charging strong enough at 500 RPM,
and is there an easy way to adjust the Idle on the fuel injected
460 (1991 model year)?

I know it makes no difference, but it's actually a former F350
Crewcab that has been converted into a 4-door SUV (Like
an Excursion) by a company called Centurion (Picture below).

 

You can't adjust the idle on a fuel injected motor at all. The IAC should be right on the throttle body on top of the motor. It may be gunked up, which can cause a lack of air at idle, as the throttle plates are basically closed, and the valve is the main source of air.

Not sure on the alternator, but any auto parts store can test both the battery and the alternator for you.

 

The alternator is new, but as with anything there is some tolerance in the
adjustment of the built in regulator, and I was wondering if it's designed to
cut out when the RPM gets low (either based on frequency or amp output).

The Battery is almost certainly bad. It's at least 4 years old, and has been
through a lot lately (electrical problems, including the blown
alternator/regulator as a result of crossed jumper cables).
As soon as RPM gets low, the voltage drops from 15V to about 8V.

BTW, if you have a black terminal cover on your positive cable...
I'd suggest making it red somehow. I just took mine off all together.

On my Suburban I had covers on both cable ends... black and red.
I saw the black one and didn't double check (in a hurry), luckily
only one wire fried significantly. I suspect the alternator failure soon
after was related as well.

 

It should adjust the idle all by itself. Have you checked it for codes or done any tune-up work? Does your check engine light work? It does sound like it is idling a little slow, but I can only guess. Does it idle-up for a cold-start?

 

It does idle up when cold, then slows a bit too much when
it reaches operating temp.

The Check Engine light works fine... gues how I know it works...

I need to replace the O2 sensor. It's on my list... honest it is!

But the Ck Engine light has been on since I bought the truck
a few years ago, and the slow Idle is fairly new.

I had it tuned up, but i don't want to take it back to the
mechanic. I don't know if he read the dignostic, but
I will take it to a dealer next time... probably just once to
get everything stabilized and working right.