I put these in both my '37 1/2 ton(converted to 55-59 open driveline), and my '59 1/2 ton Napco 4wd conversion(both ends of course). The difference is certainly noticeable(more so in the '59 because of the taller tires) but don't expect miracles. You will probably get 7-10 more mph in high at your cruising rpm(about 2200 rpm for me). A better option is an overdrive tranny. They are getting hard to find though. With either the Borg-Warner or Saginaw, and your stock 3.90 rear end, you effectively have a 2.73 rear end ratio in overdrive. You can also split the gears with the overdrive(but first w/od is almost identical to second wo/od). Second to second w/od, to third wo/od is handy though when pulling a heavy load. I think the ultimate in this respect is a Saginaw four speed with the overdrive unit from a three speed grafted on the back end of it. Going to try to get one of those installed in my '37(presently has SM-420 in it).
