I put about another 50 miles on the 348 in 90 deg weather. I could be wrong but I think this is what the deal is. I think I have a water capacity problem.The motor holds like 4 gal of water or something like that, and the radiator doesn't hold enough cool water to cool down all the hot when the time comes. Like trying to cool down a gallon of 200 deg water one cup at a time. That's if the water had time to cool in the rad b/f the thermostat doesn't open and creates a loop of hot water. The motor will cool just fine for about 6 miles or so and if the thermostat stays open during one cycle, it's screwed. If I drive 55 and watch the temp and as soon as it hits 200, I slow down to about 45 and it will cool it down. I took a heater hose off and the pump seems to be pumping like crazy and the air getting pulled through the radiator with the fan is freaking hot the whole time its trying to cool until it does cool down, then you can feel the temp difference during the cycle. That tells me its pumping good but the I just have a lot of water to cool. I pulled the shroud off and that helped some down the road and hurt some in town. Like I said, I think that's what is going on, not 100% positive but that's how I read it. I would love to try a made reverse flow pump or change the impeller back to standard flow and v belt it but I think tuning the page to a more driver friendly motor is better time and money spent. I am hoping to sell this motor and trans together and cover the cost of the 5.3 swap, and maybe put a little in my pocket