Sinky, Is this the article you refered to where Dave Bean is mentioned ? it also gives a picture of the mod.
QUOTE:
Bean also recommends an additional modification to improve the performance of the oiling system. A restrictive plug with a 0.090-inch-diameter hole can be installed in the block to reduce the flow of oil to the head. In a Twin Cam, the valve train receives a disproportionate flow of oil when the engine runs at high revs, causing a build-up of oil in the head. Reducing the oil flow to the head allows more oil to be retained in the sump, and also provides additional flow to the crank and rods