Roger "the fast one". And the RPM's you are pulling are at at the top of the limit. Personally, I would not want to run that engine in that range very often..otherwise they go Kablooey. You might...I repeat MIGHT...be able to go a tooth or two taller on the rear end, but that will come down to what axle & gearing you already have. Other than that, taller tires might give you maybe 3mph.
60+ plus in a school bus is pretty close to as good as it gets except for a few "special" rigs that were set up at the factory for higher cruise speeds.
And don't forget...going faster equates to the need for more stopping power.