What I would do is pull the distributor and with a long screwdriver turn the oil pump and make sure you have oil coming out of the rocker arms. This 'drilled bolt' is the center one on distributor side as I mentioned, you could pull it out to see if it has this small hole in the end, hole exits up on the side above the thread. Hole is under 1/8 inch. Again not sure what year your 261 is, different year motors accomplished this oil route not using the drilled bolt.
Very nice job on the chassis. Post more pics of the truck.
Jim
