Joe C,
If you can reach the listed maximum permissible road speeds in lower gears, then you know it isn't the governor holding you back. Sounds like you have already changed your fuel filters so you know that isn't the issue.
Does it start easy and have immediate throttle response when you rev it up? If not, it may be sucking in air as others have suggested.
Have you checked or changed your air filter? If you look down in the air filter base with the air filter canister removed, there is a screen wire over the inlet to the blower. Be sure to check this for any foreign objects that might be restricting air flow.
Have you checked your throttle linkage to ensure that the throttle lever on the governor is moved it's full range of motion when you actuate the foot throttle?
If you have done all of the above without any success, have you adjusted the valves and set the injectors? Do you know what size injectors your engine has in it? It should have N50.
Does it smoke much when starting off or when accelerating while running through the gears?
Out of curiosity, is your engine the one originally installed in the factory and has it been rebuilt? There is usually a data plate on the passenger side of the valve cover with rebuild info if it has been.
Good luck,
Joe S