To add to the problem description and some of the stuff I've tried:
The misses, stalls and backfires tend to happen more when under load (while accelerating with the pedal down, even part way down, climbing a hill, etc.) I don't have the problem when driving a constant speed on a flat road (regardless of how fast, and how far down I have the pedal pushed).
When the misses/stalls do occur, it can be pretty dramatic. It feels like the engine dies for just a moment (more than one cylinder misses in a row) and it feels like the truck is going to stall completely. But after another moment, things kick back in (may get a backfire or two at this point) and the engine runs fine again. This can happen many times while accelerating. Once I start cruising, even if it's a high rpm cruise with the throttle opened up, I no longer have the problem.
The problem slowly diminishes after the engine is warmed up (~20 to 30 minutes of driving). Once warm, I have zero symptoms and can drive the truck all day at any speed and at any acceleration without any hesitations or misses.
Originally I thought the carb was getting starved of fuel, but after swapping out the fuel filters and making sure the fuel lines are clear, the problem persists. I checked the carb float level when I rebuilt it. When the problem happens, there doesn't appear to be a shortage of fuel. The problem is just as bad (and just as frequent) with the choke closed completely (running rich) as it is with the choke open. I think I've exhausted all the possible problems with the carb.
I think I need to snag me a vacuum gauge and test cylinder compression. I hope it's not a blown head gasket, but I'm running out of options. I also want to check exhaust back pressure. The muffler on the truck is pretty nastily rusted and I wonder if it's clogged with rust. Still, neither explain why the problem only happens when it's cold...
