I too have searched for a solution. I have also looked underneath the truck, replacing transmission is easier said than done.
Got to thinking.
To make engine run slower at speed, the ratio of engine to final drive needs to go down, so...
A small gear multiplier mounted to the input side of the Rockwell's would do it. Given the ratio is going up, torque would go down. Building a small gear multiplier would involve two gears-big one and small one. These two gears would mount in a housing-even a built up housing made of carbon steel. It would function much like a Hallibrad quick change. Gearing could be easily changed for optimum engine speed. Ouveson offers a complete gear change set up, but is limited to a steep ratio change. Add on small gear multipliers could mount right to axel-there are plenty of spaces to secure it to pumpkin. Oil could be circulated trhough it with a hose to main axel housing. I will look at box and rating size to do this and see if it is viable.
Pete