Oil pressure wise a 261 shouldn't be different from a 235. So warmed up 35-40 lbs is good. 15 lbs or so at idle. I use the old 10 lbs per 1,000 rpm. Yes try another gauge first. I doubt that you would lose that much pressure at the rocker shaft without totally flooding the top of the engine. There could be a leak where the pump feeds into the oil galley in the block, a weak or stuck bypass spring, a bad pump, a clogged line to the gauge, or Heaven forbid excessive clearance in one or more bearings. Take it a step at a time and eliminate the simple stuff first. Don't run it with no pressure. Good luck! It sounds like a fun ride.
