Interesting question.
Ignoring brakes and steering and the condition of the suspension and tires and all those kind of things, I think you're getting into discussion of bearing durability and drive shaft speed. I've seen some online calculators that deal with drive shaft speed but never one that deals with bearing durability.
But you are ignoring a lot of very important factors there.
My own bet is that you'll be pushing the 235 harder than it will go before you'll exceed the abilities of the Eaton HO72. The Eaton has been used in racing applications, but that was mostly years ago, and today they aren't too common there.
You might want to start by looking for the 4.57 or 4.11 gears available for the Eaton before spending money on a 6-speed. Some of the cheaper overdrive 5-speeds or 4-speed might be a more practical choice. But the most common 5-speed swap isn't stout enough to support the loads your 1-ton was designed to carry, so swapping something like that in might be asking for trouble.
The racer crowd can accept failures and service intervals that don't make sense for those of us who need our vehicles to work every day. While what they do can be interesting and instructive its not often the best approach to copy.
In good condition that rear isn't the first part I'd worry about. I'd start with the tires and brakes and steering. But we're ignoring those for the moment...
