The rear gearing really depends on the engine, I have found the diesel engine needs the stock 3.90 gears to do hiway/freeway speeds, and you will not do well mileage wise if you want to go 70mph.
As I said, the engine is what really decides the rear gears, my lighter than a 720 Datsun 521 kingcab has an SD25 in it, I put 3.50 gears in it and it was a good choice for that engine, it would go down the hiway just fine, but any significant hill was an issue, I would end up dropping at least one gear most the time and end up at 45/50mph floored which in my head sucks.
The Datsun 720 dually with the SD25 turbodiesel has 3.30 gears, but that injection pump was professionally turned up, and the stock intake vane in the turbocharger was removed and a larger vane was installed, that engine has a shitload of torque, it can turn the 3.30 gears from a stop even pointed up hill with a load/towing another vehicle.
Now my 521 kingcab has a turbocharger also, but the injection pump is stock, and the turbocharger has not been modified, I really have no more power starting out, the turbocharger kicks in after I am moving, I would like to try 3.30 gears in it, but when I tried starting out in second gear with the 3.50 gears I decided that the gears I have are fine, if I run across a set of 3.30 gears I will buy them and try them out as I believe they will help me on the hiway/freeway, but starting out in first gear will be the deciding factor on if I will keep them in the 521 truck or go back to the 3.50 gears in there now, fact is I can go plenty fast, but 3.30 gears are rare and very hard to find so I will grab them if I have the chance.
I had 3.70 gears in my dually 720 when it had the SD25 with the inline injection pump and the rebuilt SD22 with the VE type injection pump, it struggled to do 70mph with both them engines although the SD25 had veggie oil put though it before I bought it, it didn't have any more power than the SD22, I was not happy with either of them engines in the 720 dually truck, but the SD25 turbodiesel moves it just fine.
I believe all the SD25 engine comes with turbo pistons, or should I say that Larryr's research into the subject lead me to believe that they all have turbo pistons, but the guy on here with the turbo/bomb thread made his using an SD22 and he said it ran fine, I only found that guy because he was selling his rebuilt turbocharged engine on the Portland OR craigslist, and after driving my 521 kingcab on that trip and finding out how cool it ran(no inter-cooler), I believe that heat isn't going to be an issue on most hiways/freeways except when going up mountain passes, that is why you will need a EGT gauge, you do not want to be heating up a SD22 to 1400 degrees, your going to have to research this subject and pick a number, and believe me when I say that a stock SD series diesel engine will get very hot when going up a hill, I seen 1400 degrees more than once since I installed an EGT gauge to see just how hot(EGTs) a stock engine gets.
I have read that at 1400 degrees it is not if it will melt down, but when will it melt down, this very subject is why it took me so long to get both my turbodiesels on the road, the 720 ran so hot that I didn't even drive it, if you read the whole 720 thread and look at the dates you will see it took me a very long time to get that truck sorted out, but I did it as a draw thru and blow thru, I like the blow thru a lot better, but I expect I could have made it work as a draw thru with the proper turbocharger that had the proper seals in it.
I sure hope you guys like reading, as I don't know what a short reply is.
