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Have you tested your thermostat on the stove in a pan of water with a thermometer? If it only gets to 200 when climbing a hill, I think the thermostat is opening too early. Very common for thermostats to not do what is advertised.
I understand about wanting to keep the tonneau, I have a bakflip on my 2001 2500HD. Have you considered building something like the off road sports bar they are putting on Chevys. It could mount on the top of the bed rails, to not interfere with your tonneau. You could style it to look like a superfluous off-road accessory, but use angles and slopes to make it lower your drag coefficient. Make it 12" long on the top with a slight downward slope, and make the sides angle back as they go down to about 24" long where it meets the bed.
My 1994 Diesel Suburban has 4.10s and a swapped 0.643 OD in the NV4500 running on 31.8" 235/85R16s. It is way overgeared even for the NA 6.5, I can lug up a steep hill at 35MPH in overdrive turning less than 1200RPM, much less 55 on the highway. I want to swap to 3.42 in the differentials, which is the lowest factory ratio available.
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