By the sounds of your test drive Dr Charles, I think you have air bubbles in the system trying to work their way out?

I just did a water pump 2 weeks ago on one of my Magnums and it took a 40 mile drive to work it all out, while the temp swung wild: it settled down and is consistent now. At first, I just went 10 or 15 miles and it was taking antifreeze and running like a stuck thermostat, but the long drive worked and antifreeze is sitting at the same level.

Perhaps others more knowledgeable can correct me though?

Oh yeah, turn your heater on WFO and crank the windows when/if it climbs in temp.