70 has a problem with the temp creeping up

I throw my 2 cents in. It sounds like your have a water flow from the block to the radiator problem A radiator is a heat exchanger. The fact that it cools at higher rpm tells me your system is lacking in water flow. I like to place a radiator thermomator in the top with the cap off and see what the temp difference is bettween it and your gauge(a mec. gauge), it should be lower but not alot lower. Next i would have thr radiator flow tested at a radiator shop to make sure the radiator is capable of doing the job. A good radiator shop can tell you this. I assume the pump belt is tight? Next you can go to a A/C type pump as it has two extra impellers, you can speed the pump up by using a smaller pump pulley or larger crank pulley. Next is long shot, but you may have so much sedament in your block that water is not getting to all the surfaces. I run a 360 with an original 69 273 A/C radiator in my Dart and it runs cool all the time. I'm fixing to put A/C back in it so we'll see what happens. Good Luck