If you want to know if you can go longer on oil changes, I recommend you wait till the next time you change your oil and take an oil sample and have it analyzed. I use Blackstone Labs
. They sell a vacuum pump for $30
that allows you to take a sample without changing the oil. They can tell you if it's safe to run the oil longer.
Also, there are some higher quality filters out there than the standard Motorcraft FL-1995, which isn't to say the OEM Motorcraft filter is bad, just not the greatest. Fleetguard makes some units designed for larger trucks that filter better, as does Donaldson. I use a Donaldson ELF7405. Here's some details on it.
They can be had online for less than $30 each.
Also, the surest way to increase oil change intervals is by adding a bypass filter, but doing so without oil analysis is a gamble. The 7.3L uses a secondary oil pump that drives the fuel injectors. As a result, poor quality oil can contribute to a poorly running 7.3L more than other diesels.
Removing the "governor" would not help you any. The governor isn't governing vehicle speed, but engine speed, which in turn limits the vehicle speed when you're in your highest gear. Allowing the engine to turn faster can result in a broken valve-train, or a rod through the side of the block.
It sounds like you don't have an overdrive transmission. Do you know what transmission model you have? Is it a manual or an automatic?
If it's a manual, you may come out better finding a transmission with overdrive to improve mileage, and that would come from lowering engine speed, not increasing vehicle speed.
If the truck is loaded pretty heavy, then lowering engine speed may increase exhaust temperatures, which could be a bad thing if it goes too high. I'm not familiar with the 7.3L in F-650's, but I guess it has a larger air cleaner, larger intercooler, and a less restrictive exhaust than a F-250.
My volunteer Rescue Squad has a 1995 International with a 7.3L, but it's labeled the T444E, and it too stops at about 61 MPH. It has a substantial intercooler, puts the Ford F-250's to shame. It has an Allison AT545 transmission, which does not have overdrive. Their latest truck, is a Ford F-750, with a Cummins ISB-360, mated to an Allison 3000-EVS 5-speed transmission. That truck tops out at about 70 MPH, because it has overdrive.