John, if the pressure reading dropped within a week or so, I'd look closely at the electric sender. Decades ago I had a friend that worked at an auto parts shop. I borrowed a dozen stock NEW senders and tried them one at a time on my engine. I got ten different gauge readings that varied by +/- 20 lbs. I picked the one that was closest to a mechanical gauge teed into the block and returned the rest. I now use the mechanical block gauge if I want to know the true oil pressure; the electric dash gauge only tells me I have (some) oil flowing. Incidentally, the electric water temp gauge reading is also unreliable.
You'll get a higher pressure reading if you add a sender at the front of the block near the fuel pump. That oil comes right out of the oil pump, compared to one in the (stock) rear position, which reads pressure after the filter, all the engine bearings, 16 lifters and any random leaks. Be careful comparing pressure reading with engines rigged differently. A bent or thrown pushrod or lifter out of its bore will notably drop oil pressure. I've found these even on low-rpm diesel engines.
I'd also look at the single wire that feeds the gauge, and the gauge's panel's ground. Odd things happen with the console gauges when the ground wire connections get slightly loose. Rarely is the stock pump to blame. The standard 351-C pump is larger than the ones used in the 427 Ford GT-40s at LeMans. Another rare happening is, the thin-wall oil pump feed line in the pan cracks where it screws into the pump body, so the pump delivers oil and air. Some shops spot-braze the line to the pump or add a brace. Good luck-