Do you have a 1/2 ton? If so, it should not be a problem to run 60 with a 235 high pressure engine, although 55 is a sweeter spot. Could be that you have an engine or radiator problem. Best to investigate that possibility before you dive into a major redo.
Here is an image of the gear set that Patrick sells that ought to get you up to 65.
3.55 gear set from Patrick's Here is a link to Patrick's web site:
Patrick's Good luck!
Kurt
