After re-assembling the gearbox it is time to get into the almighty engine. The oil leaking down the sides of the pan seemed a good place to start... hoping that I could reach the oil pump bypass check.
The level of grime was about right for 50 years of non-detergent oil. I didn't find any big chunks of metal, but there was grit and a few small shards of something.
There are drawbacks to the non-detergent oils of old, but at least the goopy gunk does settle to the bottom.
I hope that none of this migrated into the oil pump. I'll be pulling the check ball next since it is accessible without taking the timing cover off, and then I'll gap the bearings. If those both look good then there's no smoking gun to the low oil pressure.