I have my '60/'61 Factory Light Truck Manual and the oiling for the rockers feed off of the front and rear cam bearings. through a galley right at the bottom of the side cover, then up to the bottom of the headbolt hole and through a 1/16" orfice to the adjacent rocker oill feed on early models or a 11/32" hole on '59 and later engines. The rocker feed hole appears to be a blind hole, but the orfice is in the side toward the rear of the engine. You should be able to find it with a piece if small wire bent 90 degrees to put down the hole and see if you can hook into the side orfice. Make sure the wire is less than 1/16". This is all in the block. The oil path in the head should be clear through the head.
I also have a picture of the Full Flow oil filter on the 261. Its connections are like nothing I have seen on a 235, and they also have the note that "If oil filter lines are removed, connect a jumper between the fittings on the cylinder block. Do notplug fittings in cylinder block as this will cut off oil circulation". This is on Page 8-5 of the 1960 Chevrolet Truck Shop Manual
