2006 GMC Sierra with the 4.3. No power, had to be towed, only code is 0300 random misfire.

I was going down the interstate at about 72 mph and all was well. Then the truck
started loosing power. By the time I got it off the interstate (about 1 mile) Foot to the
floor, doing about 15 mph. Black smoke coming out the tail pipe. I had to have it towed.
So far I've changed cap rotor, wires, coil, air filter, two O2 senors, and the ECT sensor..
Still the same problem. Idles rough, but starts right up. Fuel pressure 55 with engine
off, 62 while running, doesn't lose pressure when shut off (goes down a little to 53
when off). What would cause this problem?