Volts Are Up, Oil Pressure is Down

[BrentBTK]

Hello all. I installed new Dakota Digital gauges in my 71 GMC I just picked up. I'm still working through some speedo and fuel issues, but the pressing thing is what I noticed on my first real drive of the truck yesterday after putting them in. When running down the highway between 2-3k, my voltage swings from about 15.5 to almost 19, spending most of its time between 17 and 18 volts. That seems awful strange to me. The other issue is the oil pressure. When running down the highway, its a strong 60 or so psi. Sitting in traffic with it in gear it drops as low as 9. If I put it in neutral to bring the revs up, it'll hang around 20. That makes me a whole lot more comfortable than 9... Does anyone have any thoughts? Should I start looking at alternators and hit the panic button on the engine? The oil pressure sender is mounted on top of the block behind the intake manifold, which I thought was odd. Thoughts? Thanks in advance!
BTK