Welcome to CHR!
Love those early F100’s. Classic design and not a whole lot of them left. Glad to hear you’re working on one. As to oil pressure – it’s not at all uncommon for a cold small block Chevy (SBC) to have fifty-sixty PSI of oil pressure and then drop below 20 PSI when it’s all warmed up. When they’ve got a few miles on them, things loosen up and oil pressure will drop. If the pressure stays above 20 when you’re driving (above idle speed) you should be okay. A high volume pump will move more oil, but not necessarily at a higher pressure. The pan sounds a bit funky and I’d recommend that you swap it out for a stock pan. Key here is to make sure the oil pump, pan and pickup are correctly sized for your engine.

I’ll go out on a limb here and assume that your engine’s bottom end is stock. As such, the following replacement components from Summit will work just fine.
Oil pan:
If your dipstick is on the driver’s side, MIL-30700
If your dipstick is on the passenger side, MIL-30701

Pump w/pickup:
SES-3-60-08-005

Make sure the block mounting surface is absolutely spotlessly clean. Get a new gasket and use a bit of gasket sealer. This is a bit tricky to get the distributor drive shaft in place and the pan sealed up so if you’ve not done this before, get some help or get a SBC assembly book that has a description of how to change the pan.

Have Fun,
Glenn