Before you decide to remove or replace the engine, try to get oil to the rockers. There are several things you can try before removing the engine.
Remove the side (pushrod) cover on the passanger's side of the engine. Its held on with 10 7/16" bolts or flat-head screws. You will have to remove the distributor and ignition coil to get the cover off.
The metal oil line we're talking about will be right there under the cover. Carefully remove the line from the block fittings. Wipe the oil line off, and try to blow air through the line to see if its plugged. Let the line soak in a bucket of paint thinner or parts cleaner.
Now, while the line is soaking, you can drive the oil pump with a drill and see if oil squirts out the lower fitting. The oil pump on these engines is driven by the bottom of the distributor shaft. The camshaft turns the distributor shaft, and the distributor then turns the oil pump.
Remove distributor (if you haven't already), and insert a screwdriver shank or oil pump prime tool into the distributor hole. Run the pump clockwise for a minute or two. Oil should squirt out of the lower fitting that connects to the line you removed. If you get sprayed with oil, the bottom of the engine is OK. You know the problem is either in the line you took off, or the passage in the cylinder head. If you don't get any oil, then the problem is in the bottom-end of the engine.
Blow compressed air into the upper oil fitting on the block. If the air comes out the feedtube in the center of the rocker assembly, the head passage is not blocked with crud.
If no air passes through, you'll have to remove the rocker assembly and blow out the head passage with compressed air. Dig the gunk out with a straightened paperclip and some carb cleaner. Then reassemble rockers & oil line. Run pump again, and oil should flow to the rockers.
Sorry for the novel, but this will help you figure out what the problem is. In case you haven't guessed yet, owning one of these trucks is very labor-intensive. You've got to have a decent set of tools, and the desire to get really greasy. Very often. The reason this BB is here is because the owners (us) have to work on these trucks ourselves. Most of us can't afford to bring our truck to the shop every time the engine blows up or a wheel falls off. (Hey guys, she thinks I'm joking)
Good luck!
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