I did the flush to insure that all of that oily gunk was out of the car to prevent plugging the new radiator. The old one was partially clogged.

I had thought of the power band thing, which was what caused me to try driving in 4th. My only concern though is, that the temps will still rise in 5th even during conditions of very light to no throttle. On flat ground the car should be able to propel itself in top gear without undue stress, even an underpowered one.