My truck looked like this when I bought it.
I cleaned the valve train as best as I could using a brush and solvents (gas, carb cleaner, etc.). I plugged the oil return holes with a rag that nothing would float down towards the pan.
I got it really clean.
Check your PCV valve. I bet it crusted shut. Also, you'll probably need to replace the tubing/hoses that run from the PCV valve to the throttle body.
This worked wonders: Hook battery tender up or keep charger handy....you'll need it. Keep filter on. Drain oil, reinstall drain plug. Disconnect spark plug wires from plugs. Fill crank case with with a 50/50 mix of kerosene and paint thinner, as if it's oil, except over fill it by 1 quart. Let the van sit over night. Crank the van over for 10-15 seconds. Do it again 2 or 3 more times. Let it sit for an hour or two and charge. Crank it over again for 10-15 seconds 2 or 3 times. Recharge battery. Do this cycle 4 or 5 times. Drain the solvent mixture from your engine. It will be BLACKER than coal. Replace filter. Refill with any HDEO you want.
I also did several short OCI's on the truck with some 20% MMO (use a grade thicker oil) and then ran for 10-15 minutes with another quart of diesel fuel in the oil to help clean it out. This helped circulate the solvent (diesel fuel) through the engine and clean up more crud.
This is what I did with good results.
People might be scared to do this because of "knocking a chunk loose". But I did this and it really cleaned up my old abused 350.