It looks like everyone has already mentioned what I was thinking, but if the ignition timing is retarded, it will run hotter. Same with a plugged exhaust. I don't think that either would heat so fast that you couldn't complete break-in though. Second, get an infrared temperature reader and see if it is really hot, or if it is a gauge issue. As an aside, my Champ had the wires to the gauge reversed, and within minutes of starting, the gauge would peg, but then soon drop. I never understood why it would act that way, but once the wires were in the correct place, it acted normally. My point being, don't rely on the gauge only. Third, I like the idea that the racing water pump may be designed for serpentine belts, thus runs backwards with conventional belts.
Keep us informed!