Some years back I had a 51 with a rebuilt 235 in it that ran really well in stock form. The only other things I did to the truck was to swap out the rear axle for one with a higher ratio (3:73 if I remember right) which allowed the truck to cruise all day comfortably at 65 mph and then disc brakes for the front axle to scrub off some off that new found speed.
I'm a dyed in the wool six cylinder guy so of course I'll also say keep that stovebolt six if you can.
