Your mistake was installing the 0-60 lb. GMC pressure gauge. There is no such thing as a "high volume" or "high pressure" oil pump for a 216. Pressure to lubricate the rods on a spray-oiler engine is created by the speed of the crankshaft dippers spinning against the streams of oil squirting across the crankcase from the spray nozzles. A "good" oil pressure reading for a 216 or a spray oiler 235 would be somewhere in the 10-20 PSI range, and that will be at highway cruise speed. Idle oil pressure on a hot engine will barely be off the zero mark on the gauge. At idle the dippers on the rods swing down into the troughs below each rod, so as long as there's at least a trickle of oil coming from the spray nozzles to keep the troughs filled you're good to go. The main and cam bearings and a small volume of oil to the rocker arms is the only part of the system the pressure gauge is involved with. If the rockers are getting oil, it means the pressure in the lower end is supposed to be at least 15 PSI. The system is designed to cut off oil flow to the rockers any time the main and cam bearings are in danger of losing lubrication due to low oil pressure.
Adapting the 250 pump to the 216 will probably end up with hot idle oil pressure somewhere in the 20-30 PSI range. Max pressure at cruise speed will be determined by the pressure relief spring in the pump. As of yet, I haven't gotten an engine with the different pump in running condition, since that is only one of many modifications I'm doing to a "216" that will end up having a displacement anywhere from 235 to 258 cubic inches, depending on the stroke of the crankshaft I'll be using. It will use a modified 235 crankshaft with full pressure rod oiling and hydraulic roller valve lifters, among other modifications.
Jerry
