A M935A2 will do 60mph. Whe you are operating the vehicle on the highway you need to run at least 60psi in the tires. If you have the CTIS, you will have to disengage this. It tells you that in the 10 manual. It bounces, but i have traveled in those with no issues. Modifications?
Well of course there are, but that is for another post.
The 915/916 variants are nice, but just don't have that mean look to them. They can do 60mph. Are they civilian or military?
The M52 series of vehicle wer just to bulky, though well equipped, powertrain wise. They are similar to the M35, which even with the A3 version, of today, it still struggles to get 50mph.
The 800 series are older, but they can be ran up 60 with minor tweaks and cleaning of the system.
One other thing is that the M35, M52 variants can all handle super singles, which makes the ride nicer.