22r Carburated, 327,000 miles. The truck has a two year old front engine cover and gear set as the timing chain did eat a hole into the water pump housing. At that time the head was rebuilt and a shim was installed to raise the deck height and help prevent the problem. There are none of the symptoms of a tensioner failure like slapping chain noise in the front of the engine, or oil in the anti-freeze.
I can check the compression of the cylinders and look at the valve adjustment to try to find out if they're totally wacked out, but I can't imagine either would be enough to make it run this badly.
On my way home tonight, I was coasting downhill in third gear with the throttle off and the backfiring was constant, and that I think is the "crux of the buisquit."