Bill, are you sure a 55 would have the 1/2 ton axles? I thought the 55 and 56 1/2 ton Napco conversions used a 3/4 ton 8 lug axle up front.
Now again I was wondering about the comment of the truck screaming at 45mph. Is it possible the transfer case is in the low range? Seems to me a truck with 3.90 gears and about a 30 inch tall tire should have no problems doing 65 mph at about 3000 rpm. I have a 59 2wd 1/2 ton longbed with the 3.90 rear gears. When I got my truck it had a tired sm420 which I have since changed out to a NV4500 from a dodge 2wd. I used the dodge version because it was eazier to adapt to my chassis, and it can be modified for a mechanical speedo. You won't have the problem with the speedo because it will hook to your transfer case.
