I think the PO put stronger springs in the oil bypass valves. Just thinking about it, this seems like it would be a mistake, and the factory had it right. How do I check this, and am I right?

The HD oil pressure relief springs have been around for a long time. I don't think it's a bad idea, but you can accomplish the same thing by shimming/strretching the existing springs.
I had a low pressure problem with my 2.2 with a Melling oil pump when the engine was hot. Stretching the spring in the secondary relief (driver's side near pushrod tubes) brought the pressure up a bit across the rpm range. The cast iron Melling pumps are known to have oil pressure problems, partly due to the lower expansion rate of cast iron compared to aluminum, which creates leakage at the pump body.