What do you mean by it feels like you're pushing it too hard? does the engine get rough at higher RPMs? loud? shaky? OR are you just not used to a vehicle that operates at a higher RPM than most?
To me the Gen 1 is comfortable at highway speeds and takes 75mph like it doesn't even care compared to trucks of the same era, this characteristic is what made the Montero stand out. Remember 4x4's (especially boxes on wheels) were typically not so great over 55mph and in the early 80's 55mph was the national limit while the late 80's upped it to 65mph. Compared to SUV's 10-20 years newer than yours i think i'd understand feeling like it's being pushed but 80's for 80's 4x4 on the highway you're driving a rocketship. If you want to bring the RPM's down a few hundred get some 33's, all in all the Gen 1 LWB should be smooth and quiet at 65-75mph cruising between 3,000 and 3,500 RPM.