This sounds like a really interesting project. I'd like to travel faster too in my 3800 with camper but I'm unwilling to pay the fuel bill. I run right around 60MPH at engine 2850RPM. The brownie has a 17% overdrive and 24% underdrive plus direct. Gas mileage before 10% alcohol was added was ~11.5 MPG. Now it's barely 10. As I travel east from Washington State gasoline prices go down and speed limits go up. When I hit Idaho it's 75. When I hit Utah for a while the sign said 85 but now it's 80. Montana now has a speed limit on interstates, 80MPH, at least on I15 north of Idaho. Last month when I drove to Salt Lake City air temperature was around 100. I was concerned about tire temperature and held speed below 60. I'm right at the limit of tire capacity at max inflation pressure. On interstate 84 there were many alligators (chunks of truck tire) to dodge in the slow lane particularly at the bottom of hills. Guys blow by me in the hammer lane like I'm standing still; then an hour later I pass them sitting by the side of the road with the triangles set up. It's a long way from Sublett, Idaho to the nearest tire repair shop. Super singles are not your friend out here.
So aside from oil starvation in the rear axle, there are other considerations to traveling at modern interstate speeds. I added oil temperature gauges to the engine, brownie and soon the rear axle. Big trucks (class 8) have such gauges and they are not to be ignored. Climbing mountain passes temperatures go up by 100 degrees or so over the flats, much more than that and you've got bearing beginning to fail. Mind you these are manual gearboxes. Automatics are a different story.
