I had my 52 1 ton pickup setup with 4.56 gears, 8R19.5 tires, SM420 transmission, and a 1955 model 235 engine, truck weighed 4,400 lb empty.
I drove it round the country, from dead flat to big mountains, and mostly unloaded although we did have several hundred pounds of gear/tools and extra fuel. Speed was limited by the slow gears 98% of the time, on some of the really big mountains I ran out of power and would down shift. We traveled between 2,200 rpm and 2,500 rpm, at between 50-55 mph, occasionally faster. I would have really liked 4.11 gears, and probably even 3.73, although with the 3.73 gears a direct drive 5 speed with a close 4th and 5th ratio may have been helpful.
If you know you will keep the rear axle you have found, figure out what gear ratio it is, and give it a try, if you feel like it needs faster or slower gears, or a different transmission, then take that step once you get there.
Make a reasonable guess with what you have already, try it, and recalculate and swap things later.
Just an example; I assembled a diesel truck and calculated that 3.73 gears, 0.80 OD, and 38" tires would be just perfect, once I got it together I drove it on an 8,000 mile trip (two weeks), and not long into the trip came to the conclusion that 3.54 gears would have been much better than the 3.73 I used.
Point is you don't really know till you try it.
Grigg
