Hey '58 stepside,
I also have a '58 stepside (3100) 235 c.i.d., 3 spd, stock 3:90.1 rear end. I can cruise at 55 - 60 easily all day long.
What size tires are you running? What kind of noises is your engine making? Are your valves set correctly? Be sure to check for other things like bad bearings in a gen (or alternator) they can make a heck of a racket too. Let us know!
