All my plugs are fouled?

[Smniekamp]

1981 Chevy K20 350 with 4 speed manual 4x4 (70,000 on rebuild) stock.
Hey guys,
I was driving down the road when all of the sudden I felt a miss. The truck hesitated then kept running, about 200 yards later it started to miss really bad. It felt like a automatic trans that was about to die, slipping in and out of gear (however I have a manual). I ended up having to tow it home. It will start and run very rough, stumbling hard and missing. I can step on the gas and bring the rpms up to around 2500 and it will run a little better (less miss fires), but then will just completely cut out and die instantly. It almost sounds like its firing on half its cylinders. I threw a timing light on it and the mark jumps around on me a little. Ive replaced the plugs, wires, ICM, Coil, cap, rotor, fuel pump, accelerator pump in carb, checked the pcv valve, and checked to make sure rotor is turning (stripped dizzy gear). Still having the same issue.... Any other ideas? All my plugs were fouled when I pulled them out initially. Could a leaky intake cause all the plugs to foul? Head gasket? Any input would be awesome! Thanks!