I have a knock in all four gears at 2500-3500 RPM. I'm thinking that it could be the fuel, maybe the power steering pump, maybe a valve lifter , maybe a bad bearing or connecting rod. The engine is a high milage base 350 with no modifications.

Noises are such a pain to diagnose when ya can't really hear them yourself...
First, let's try to narrow down the exact sound...
A "knock" is normally a sound associated with rod/main bearing damage, BUT...it can be caused by other things, such as a bad harmonic dampener(balancer), a broken flywheel, or even a bad fuel pump. A knock will normally be a muted, heavy "thud" if it is in the bottom end(crank/rods) of the engine, and a little higher pitched if it is in the upper end(piston).

A lifter makes more of a loud "ticking" noise, as does some forms of exhaust leaks. Lifter noise/tick is usually more prominent at idle, and may go away or get quieter at higher rpms.

A fuel problem would most likely result in engine "ping", or detonation, which is usually a higher pitched tap/knock when the engine is under load.

I can't think of any power steering noise that I would consider a knock, unless the pulley is warped, and banging on something.

Does your engine make this noise in neutral, at that rpm, or do you have to be driving it?
Does it make the noise for a couple of seconds when you first start it?
Adams' Apple2009-05-03 19:58:09

might also consider the input shaft on the tranny... if the pilot bearing/bushing is shot, it will allow the input shaft to flop, making a knock also... but, thats usually not specific to a particular rpm... never know.... :)

OK, I'm the big dummy. Checked the oil and I was down a quart. Added a quart and sound is gone. I know this is a 'band aid', because the real issue is still there. It will surface some time in the future, hope I'm not too far from home.

The sound of a diesel makes me think you have a timing issue, or fuel problem. That would be the "ping" I mentioned earlier. The noise is actually the piston(s) "flopping" back and forth in the cylinders, due to either too much timing, a lean fuel problem, or overheating.
If a quart of oil helped, or eliminated the noise, then that would still make me think it has a bearing problem. I would suggest an oil change, and use a little heavier oil than you normally use, at least until you can actually find the time and $$$, and "fix" the problem, which is likely going to be bad bearings. It's also possible the oil pump is failing, and just not supplying the proper amount of oil pressure. It might be a good idea to install a mechanical gauge on it just to see what the actual oil pressure is, while driving down the road. At least you would know at that point what you're looking at...
Glad the oil at least got it quiet, tho.

Actually...now that I thought aboot it for a few seconds, I might have another idea, that makes more sense....
You may have oil drain-back holes in the head that are stopped up, and not allowing the oil to return to the pan fast enough. Couple this with being a quart low, it make perfect sense that the oil level in the pan could be low enough to cause the noise, since most of the available oil is stuck up in the top of the heads....
In that case, I would pull the valve covers, and check the holes in the corners of the heads, and see if they are sludged up, and not open to drain the oil back to the pan...Adams' Apple2009-05-04 19:17:39

I'm going to get the oil changed at noon. I ordered rocker arm clips so I can adjust the valves. When the get in I will pull rockers and check the drain holes. Hopefully this can be done on the weekend.

Main and rod bearing can be replaced with the engine in the chassis....one of the really great design elements of the Corvette!!
If you have power steering, you'll need to lower the steering ram/cylinder, but other than that. it's not too bad.
Let us know if you find anything (gunk) built up in the oil drain-back holes.

How can I tell if the harmonic balancer is OK or NOK? I presume that the rubber could be worn out as the engine matches the VIN, it has 118K miles and I presume the rubber could have worn out over all those miles and the 33 years.

WHEN I HAD MY MOTOR REBUILT I WAS HAVING ISSUSES AT THE SAME RPM 3500- 4000RPM i KNEW  IT TO BE A PINING AND THAT IT WAS TIMING RELATED BECAUSE WH WERE PLAYING WITH THE TUNE. THE REASON I REBUILT THE MOTOR WAS BECAUSE I STARTED TO GET A KNOCK WHILE DRIVING AND IT WOULD INCREASE IN SPEED AS THE CAR INCREASED IN SPEED. THIS TURNED OUT TO BE BAD ROD BEARINGS. GOOD LUCK WITH THE INSPECTIONS JOEL GAVE YOU.