Let's pretend that you have 27" tires, and a standard 3.90 rear end, and a final transmission gear ratio of 1:1
At 55 MPH your engine RPM would be about 2700. A little high, but not really straining the engine.
The cheapest way to get lower RPMs and higher speeds might be to get larger diameter tires. 30-31" tires would get you 2500 RPM at 60 MPH.
