After a great deal of trial and error, I know what the howl is not. It isn't:

Induction -- filters on or filters off, no change in the tone or intensity.

Exhaust system -- I added a resonator at mid-pipe and all hangers are in place; it doesn't move around and it doesn't touch anything. There is a small exhaust leak at the manifold/headpipe junction that will be resolved when the rebuilt engine is installed.

Drive line -- replaced the center bearing carrier rubber grommets. With those inexpensive pieces of rubber, all driveline components from flywheel to pinion yoke (including the yokes) have been refurbished. (It is still possible the pilot bearing is the source of some resonate vibration; it will be renewed with the engine.)

Water pump -- just replaced; howl still present at the same engine RPM.

I spoke with walrus3 for a few minutes this past weekend. I described several of the sounds I was trying to ID. He rightly diagnosed a failing water pump. On the way home last Monday the water pump began making a "clang" at idle, just confirming his diagnosis. I called Swedish Treasures and had the pump and two front wheel bearing kits sent overnight. Expensive to ship that way but I am back up and running with a new water pump.

The water pump also resolved a knocking noise I couldn't find. FWIW, the knock seemed to be coming from the block and varied in intensity; sometimes it was barely audible and sometimes it was downright scary. But the knock was completely independent of load. I had previously misdiagnosed the sound as the cam sliding back-and-forth. That particular sound disappeared entirely with the new water pump.

BTW, the car never ran hot. The pump continued to move water right up to the bitter end.

The engine has a light knock that (after talking with George I agree) is a wrist pin. Of course the valve train makes noise too. All identifed and acceptable for the short term.

There are several other noises and sounds that I have identified and one that I have not but are of less concern to me. I would REALLY like to know what causes this extremely loud howling noise. If you have encountered a similar noise, let me know what you found.
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Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL