f it was the cam, why would it only last for 20-30 seconds?
The cam determines the airflow through the engine. It also determines the amount of air that stays in the cylinder while the piston comes up on compression (dynamic compression ratio) and how much chance there is for exhaust to dilute the intake charge. Hard to tell everything that cam needs but it certainly seems to need more timing everywhere.

So it's possible for 20 - 30 seconds the spark timing isn't right for the engine, and maybe the fuel isn't right as well. Too little timing makes the exhaust seem rich. And we can see the computer pulling fuel once it goes into closed loop. If you disconnect the O2 and force the engine to stay in open loop you might see it run rough for longer. And that would help confirm the cam is different enough from stock to be an issue. And if you can get it to run rough longer with the O2 disconnected then you can see if it runs better at idle with a little more or a little less timing.

There is surely a difference between 0 and 6. At 6 it isnt holding back like it is at 0. From what I have read online, most people dont like 0 and are happier at 8. Thats excessive to me, I am not expecting too much out of this sled.
I would sometimes set timing at 2 - 4 advance for guys that wanted a change. You could do it if the truck was running 87 octane fuel instead of the 85.5 cheap stuff. I never had to go as high as six.

I am trying to find someone with a spare MEMCAL that I can try.
I might have one here. I would want to make sure it's the right one for your truck which means, matching the code to the one you have installed. But I'm thinking this engine really wants a custom chip.

I think you should tell me you have a dual plane intake, 4 bbl carb and a HEI distributor sitting in the corner. :)
I hear ya. I've been down that road before. At least you'd be able to make changes yourself. You'll need to separate the dash wiring from the computer wiring. And you'd need a fuel pressure regulator to keep the pressure around 7 psi. Use an intake from an '86-'87 Monte Carlo, a carburetor from an '84 truck with 350, and an aftermarket distributor for a strong combination. Or change to a nice, cheap roller cam. ;)

And just to swap a story, around '94 I put EFI on top of a 302 chevy I built in a '57 pickup. I had a mix of parts that wasn't too bad but they were all old designs. Heads from an '83 305, block from a '63 327, crank from a '67 283, and cam from a 66 327. The stock computer barely ran. I paid a guy to make a chip (there weren't really any tools to do it yourself back then) and what he made worked a lot like what you've got. But the rough running lasted about 45-50 seconds. I drove that outfit all over the place like that because the guy that made the chip said my cam wouldn't work with EFI and I needed a different one. I know the cam works because I learned to make my own chips. But the final result was quite a bit different than the 350 chip I started with.