The truck's use is probably going to be moving my tractor between my house and my father's house....
I don't see this usage as real challenging for what you're working with though I may be picturing too small a tractor. Some of the key words though are mountain roads and you'll want to be sure your brakes are up to stopping the truck with your tractor or something else heavy loaded on back.
And brakes would be my biggest concern using the old axles. The '51 still uses Huck brakes which aren't quite as good as the slightly later Bendix brakes. Even a few years later front and rear suspension might make a dramatic improvement in stopping power. I assume a 6500 has a hydravac and you'll want to assure its in good shape.
You're doing two things that make the brakes critical. Hauling heavy loads (even a small tractor is heavy) and increasing the top speed. The braking power required to stop a vehicle does not increase on a linear scale as you go faster, it increases much faster than that. If you are going to haul that tractor at 60 to 70 even on flat ground between the hills I would encourage you to look hard at ways to improve the brakes.
The 6BT/12 valve Cummins is the motor I am looking at.
You don't need to read a lot of threads here to see I'm not a Diesel guy. Don't like the noise or the smell. But I do respect the pulling power they make and I'm sure you'll find the 6BT will do what you need.
Be aware that the 6BT is long enough that most installs require cutting into the firewall to make enough room. This means moving that loud and hot motor into the passenger compartment. I've seen a number of people argue and then build using the 4BT instead because they felt the power would be adequate and the compromises to make the motor fit much less.
Personally I'd choose a big gas V-8 instead so my opinion may not matter!
So as I've reviewed what I've written I think I'll also offer an approach to the brake problem.
You're dealing with a large flatbed so the exact width of the rear axle is not critical. Probably the wider the better for stability, meaning the Dana 70 HD or even Dana 80 from a dually might be a good choice if you can find one. You should be able to find these with disk brakes, but even modern drums on a 10,000 pound rated axle could be adequate.
You may find yourself going to 8-lug wheels in this process. I would not have a problem with that though others might.
If you search for olblue here on Stovebolt you'll find a thread about converting a dually front axle to disk brakes. He is converting a Task Force 1-ton but the process is going to be very similar for an AD truck like yours. Problem is he is building a 1-ton and using the 1-ton axle and spindles.
If you look up the specs on your 6500 you'll find your front axle is rated higher than that of a 1-ton but that a 4400 is rated the same as a 1-ton and in fact shares most the part numbers. You can downsize your front axle to the 1- or 1-1/2 ton type and then follow John's disk swap pretty much exactly as he did it.
Again you're converting to 8-lug wheels in the process, but you may find it easier to get modern wheels and tires having done so. And you really do want modern wheels and tires if you're going to run this rig down the road loaded and at speed.
I'm rashly assuming your tractor isn't real big. I often haul a John Deere 770 which sits nicely on my trailer and doesn't seem to weigh enough to challenge my equipment. If you're hauling some 10 ton behemoth ignore my suggestions and buy a bigger truck.
