762, please correct me if I am wrong.
The M180 petrol motor is power limited in that chassiis, not rpm limited. But the 300 SD motor has 40% more torque and also 33% more hp, at 4350 instead of 4800. 12.5R20 (335/80R20) are 512 rev/mile, 10.5s~575. The axle/portal ratio is 7.56. So at 60mph with 12.5s it's at 3870, instead of 4350. The turbodiesel might make enough hp to propel the truck at 60mph at 3870.
My guesstimate is that since the diesel has 33% more power, and power draw at those speeds ~proportional to square of velocity, the diesel has the power to propel the vehicle 15% faster. It may run out of rpms before it runs out of power, whereas the M180 petrol motor runs out of power first.
Charlie
PS If you double your investment with the diesel conversion, also invest in a set of radial tires like 335/80R20 Michelin XZLs and make sure the brakes are in good shape. Also, another limit on long distance high speed driving is the portal boxes. On a 404 they are pretty small and I think they only hold 300ml of lubricant, They can easily overheat at high speeds and require very expensive repairs. Leaks must be fixed.
