The basic idea is sound.
Try this to start with:
http://www.drivetraindirect.com/t_gear_calculator.htm If you truly don't want to go faster than 60 then it should work with an overdrive trans with 0.73 over (like the NV4500), as well as 8.25-20 tires, or 9R22.5 if you have the rims.
In addition I would consider a rear axle swap, and or a second overdrive, probably a US Gear if you have the money.
You may think 60 is fine for everyday use, but I have a hard time believing that.
I have driven my share of over geared (slow) trucks. My first truck is a 52 Chevy 1 ton, 4.56 gears, 55mph at 2600rpm. I put about 8,000 miles on it in one month, 55 mph tops, usually 50, that made for a long drive coast to coast to coast and back.
Two other trucks I bought and drove home, one form CO to VA, geared exactly the same as my Chevy, it had a Detroit 4-53T, again, another long drive in the slow lane. That truck now has 2.xx gears and works much better.
Lastly I just drove a truck with yet again the same gears, but a 0.80 overdrive, averaged 60 mph from CA to VA. This truck has a 6V53 with twin turbos, I did re gear it for 70 mph at 2000 rpm, much much better. After the gear change I drove 8,000 miles around the country in two weeks, this time at 75-80 for a good part of it, there is no comparison. Even around home these trucks with slow gears are not ideal, and fuel consumption is miserable.
If you really want to keep the two speed, either find a newer one with faster gears on the high side, or invest in two overdrives. I have seen many poor diesel conversions, first on the list should be to gear it properly, without the right gears you don't have much.
Grigg
