That was the most informative post yet. So, 35 is not your maximum speed. It is how slow the truck gets on a specific stretch of road. Which is totally different than what you started this thread with.
Ok, there is a point when you are pushing the throttle pedal and nothing happens. When you get to that, look in the mirror and notice the black smoke coming out of both pipes. Back off the throttle until you see no more black smoke. You won't slow down any. But, you will reduce cylinder head temperatures and keep the engine from cooking. Black smoke on an Indirect Injection diesel means bad things.
When you get below 40 mph and you have no black smoke coming out the pipes and still a lot of hill in front of you. Manually downshift to 2nd gear and get back in the throttle a good bit. You should now be able to hold 40-45 without any black smoke. Adjust the throttle by the smoke in the mirror still.
Picking up the engine rpm does two things. It gets the engine above its max power band where the water pump is turning fast enough to keep coolant flow up. Compared to below the 1,800-2,200 rpm max torque range of the engine with a lot of heat being built and not much cooling action going on.
It also gets the transmission pump going faster so transmission fluid is sent to and from the radiator to keep it as cool as possible while it is really being stressed.
Dropping to second gear should let you make the climb in the 40-45 mph range. Then, you just need to be patient. If that isn't your idea of fun. You have 3 options:
Banks Turbo kit. High priced but it includes everything you will need to keep 70 mph for that climb or until your EGT's get to 1,100° whichever comes first.
GM factory 6.5 turbo set up. You will be the designer of how it should fit together and supplier of the multitude of odds and ends that are needed for a complete air intake to tail pipe install. GM had several different versions of the 6.5 turbo. All seem to choke themselves out at about 2,300 rpm. It is cheaper than the Banks kit though.
Leave the M1009 at home and drive your Cummins on those drives.