[Bored&Stroked]

Regardless of your reasons for why you want an older truck (and my opinion as related to reliability of newer vehicles), a couple things to think about are:

The fact that you want to keep things "as stock as possible", yet do all the other things you mentioned is pretty much impossible. If it was my choice, I would be thinking about a few key things:

1) Overdrives are great, but only if you have enough power/torque to drive them. I would not be thinking of an original 6 cylinder, with an overdrive (or higher rear end ratio), a load in the back and think that I have enough torque to take that vehicle down the road at 70+ mph.

2) High Speeds and Brakes: The suspension, steering and braking systems on these trucks were not designed for 70+ mph. So, even if you manage to have enough power and final drive ratio to get to 70 mph, how safe will you be once you do?

3) Things to Think About (Due to the above): It comes down to power, weigh, load carrying ability, etc.. To be truthful, if I wanted a 48 truck that could go 75 mph, with a load in the back and be able to do everything you want safely and easily, I would be swapping out the drive train to more modern components -- engine, trans, rear axles, brakes, etc.. I might also need a newer steering box (maybe power steering), power brakes, larger sway bars, etc.. In the end, I would have an old truck, with a new drivetrain.

There are many engine, trans, rear, suspension options to accomplish the above - all depends on what "flavors" you like, how much time/money/skills you have and what the final 'package' needs to be.

Good luck . . .
B&S