Don,
I've got a truck with one of these engines fitted - it takes approx 100/120 secs on the flat to reach top speed, which is around 50 of your miles per hour (80 kph). I'm sure you've already checked out the air filter. There are also 3 fuel filters to check: one inside the tank (you will have to remove the large drain underneath to get at it), a glass bowl hidden tight underneath the injection pump, and finally a standard screw-in canister. It's worth blowing out the lines as well...
These units were often restricted by a (sealed?) screw fitting at the point the throttle linkage meets the pump. If you chose to unseal the screw and reset it yourself to gently increase maximum rpms, you'd not be the first to do it!
Enjoy what I'm sure is a great truck
Giles