I can think of only two things capable of that much smoke: Fuel or oil.
So for oil,
What kind of oil consumption is this engine showing?
What kind of blowby vapor is visible when warmed up and at idle with the oil filler cap off?
When was the last time you checked the CDR?
I couldn't help but notice the intake was modified and wonder if the CDR may have also been tampered with.
The only time I ever heard that kind of crackle/knock from an IDI, was in an old diesel ranger that had excess blowby. Its hard to be sure without seeing it in person, but the color of smoke seems to match too (thick grey with a mild hint of blue). Oil would build up in under the valve cover until getting sucked through the intake, resulting in a moderate runaway. In my case, it was blowby preventing adequate drainback down to the sump. Light throttle and slight downhill cruising would set it off almost like clockwork. Older VW IDIs were also prone to this.
Regarding fuel/air intrusion,
One spot that could be allowing air in is at the fuel sending unit itself. Not the pickup tube inside the tank, but the lines that exit the top of the tank. Those are either zinc or cadmium coated steel, and don't last forever. If the rear tank already rusted out, I can't imagine the front sender is in great shape either. The rest of the fuel lines in these trucks hold up very well against the elements.
