The problem is still there.

I thought I was on to something when I timed for maximum vacuum. If I advance the timing to about 25 degrees BTC the vacuum climbs from 18 to 22, the engine runs well, and it does have more power. Unfortunately, when I start off it still bogs right down under load and it pings like crazy. I set the timing back to where it belongs and ran a few more tests.

Valves - all valves open the same amount and all work. No broken springs that I could see.
Compression - confirmed at 130-140 per cylinder.
Vacuum - 18 to 19. Vacuum fluctuates between 18-19 at the manifold port and stays rock steady at the carb base.
Catalytic - pulled the exhaust off behind the Y pipe. No effect.
Spark - steady at all speeds. No timing fluctuation at any RPM (once it is at a given RPM the relative position stays steady). No loss of spark at any RPM.
Vacuum advance and centifugal advance - both work.
Vacuum leaks - none that I can find by ear with a stethoscope or by spraying carb cleaner at any potential leak point.

I did notice a lot of soot out the tail pipe, but the exhaust is open.

The engine purrs at idle and at RPM with no load. Under any sort of load the truck acts like it has a Briggs and Stratton under the hood.

I suspect the next step is to pull the intake, but I hate to do it wothout some sense that it might have a leak.

I could swap out the ECU, but since it runs fine under no load and since the spark is steady under all conditions, I'm not convinced I should spend another $50 there.

I bypassed the fuel system all the way to the carb bowl with an electric pump and a gas can and the probem is still there.

This one still has me baffled.