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The trans temp gauge on the dash is like the oil pressure gauge, they're glorified idiot lights. By the time your trans gets too hot, the gauge might move to the overheat range. Your best bet is to buy an actual temp gauge and install it. I have an Isspro EV1 series trans temp gauge and its installed in the pressure test port. Its a simple install if you know basic wiring. Part 15010 here Riffraff Diesel Performance Parts: - Pillars/Misc/Related is a simple over the steering column pod that would work. You could also get an under dash or pillar pod if you plan to add more gauages one day. Here's the gauge I have Riffraff Diesel: Transmission Oil Temp 100-280F You can get them in different face and pointer colors. This one matches stock. I realize this is listed for the diesel pickups, but the interiors are all the same.
OK, you are not going to over-rev your engine. It just will not happen. Cresting the pass with your foot to the floorboard and your engine screaming at 4500 RPM in second gear is not going to hurt anything, except your wallet. Your fuel mileage will suck, count on it. What you do need to worry about when doing this is your transmission temperature.
The easiest upgrade for a transmission cooler would be to utilize a cooler from a 6.0L Power Stroke F-250 or F-350. If you cannot get one, Hayden makes some good equipment.
In my opinion, before you change out the ring and pinion, you might want to consider trying stock size tires. You might be surprised at the difference.