I would guess that a '85 bus would run at least 65mph on the govenor....my old '76 Eagle ran 80mph on spec RPM (at least it sounded like it).
Must have fuel and air:
My even older GMC Fishbowl was gov'd to 55mph (under RPM at 1850; DD 6V71), but turned it up and would run an indicated 72mph on oversized tires. Like the old man said "when it sounds like a sewing maching, it's just right". That's about 2400+ RPM. He was right. Ran the same route 4 years, ever year a different change, every year a little faster. Final year ran the same hills at 12mph faster, minimal speed lost from flat cruising. Same load, same weather, same same.
I'd also take a gander and say that with a turbo one could install bigger injectors for a slight bump in power. If'n it were me a water injection kit would find it's way under the hood, to both keep temps down and to facilitate complete combustion during WOT (starting out or climbing hills). Would also wrap the exhaust to preserve all the exhaust energy possible. Lost heat is lost power due to lost velocity, THE key factor to turbo performance is velocity.
Also recommend to ensure the exit tract is flowing well. Diesel's flow A LOT of air/CFM. Want to be scientific? Install a back pressure tester. I didn't, but did make a quasi dual exhaust and noticed a difference, besides all the noise. Previously, at WOT, the backpressure was so great, that even at speed, the pressure was great enough to part heavy water on the road at 65mph!..! Holy Sacred Cow Bat Bird!
My last little trick, not sure if it actually helped, was to use ATF for the oil bath filters. It's lighter than engine oil (SAE 30 for a 6V71) and the theory was less resistance equals more flow ='s more power. Never got around to making intact ram air ducts.....
Nowadays one has BioDiesel to help increase Cetane among the many other benefits. The 'race fuel' for compression igntion technology (or lack thereof).
So there you have it.... A full report is expected Monday morning of what was done and scientific results to back up the theories.