I've had a few V8's built in the 70's and 80's, and they didn't do that. If the temp goes up, there's definitley something wrong. Especially for an older engine like that- they're usually bulletproof and made quite simple. Your fan may be spinning, but you have to check if the fan clutch is working. Yes, as long as the motor is running it'll free spin, but if the clutch isn't working, then it's not spinning at the same rpm as the motor. Once the motor is nice and hot, try stopping the fan with a newspaper (BE CAREFUL!). If it stops, then your fan clutch is shot.
Moreover, have you ever had the coolant flushed and changed? It sounds to me like the coolant is not reducing the engine temperature. At higher rpm's, the coolant cycles through the system so fast- if it's no longer effective in lowering the boiling point, then it won't draw the engine heat away as good as it used to.
Now, this is for the cooling end of things, I don't know how that impacts or has anything to do with the oil pressure.