Yeah, you might have damaged the sender for the stock gauge, it might have been a gas pocket, it could be alot of things. If you have the cash, I'd go for a 3 core radiator, but if not, a stock size 2 core will be fine. Oil should be changed immediately after the first run of the motor, then again after a day or two, then you can go back to regular changes. That way, any loosened gunk and metal shavings are removed from the motor. Definitely get some AutoMeter gauges, after laying a bunch of money into the motor, it's worth the three or four hundred to really keep a good eye on your engine. I just got a full AutoMeter Phantom setup myself.