--- the biggest obstacle to highway speeds will be the gearing on the rear end and tranny. Both can be modified, it's just a matter of how far you want to go. Your engine compartment would easily handle a Cummins 5.9 but a properly rebuilt, a 6 cylinder gas Chevy would pull higher gears as well.
As I recall, your rear axle is very likely either a 5.43 (rare) or a 6.17 (most common) and finding anything taller is next to impossible. You can determine the rear axle gearing one of two ways...
1. Jack up the driver side rear wheel...put marks on both the wheel and the driveshaft...then, while in neutral, rotate the wheel while counting the number of turns the driveshaft makes. That number will be your ratio (i.e., 6.17 turns of the shaft to one of the wheel = 6.17:1 or what ever)
2. The other involves removing the inspection plate on the back of the axle. Chevy stamped the ratio directly onto the flat side of their ring gears.
Either way, it is a good idea to know just what your gearing might be. From there, you can plug numbers into an online speed/RPM calculator such as the one below to determine what speeds at various RPM's you can achieve.
Engine RPM Calculator
Most of these buses top out about 45 to 50 which makes highway traveling dangerous these days thanks to the lamewits who are incapable of judging speed or distance while texting on their cell phones.
Also worth noting in this regard is tire diameter. I've seen a number of people swap out their 20" rims & tires for smaller ones to get a lower profile and hot rod look. Going smaller in diameter makes a huge reduction in top gear/speed at any given RPM. Here again, the calculator above can show the effect of different diameter tires. Handy tool.
Best of luck and please do keep the pix coming.