I must comment that the factory put higher gears than 3.08 behind the 700r4 in some applications. I have a 2.73 rear out in my shop from a car equiped with a 700r4.
Without knowing both the motor and how the truck will be used I don't know how you can answer a question like this one. And you need to know more than just the size of the motor, the cam and induction are going to be part of the equation. A mild cam and EFI means the motor will run smooth at a pretty low RPM and with the OD might give you fantastic mileage running down the highway. A bumpy cam and tunnel ram are going to mean different answers.
An automatic overdrive does not need to stay in overdrive all the time to work right. Its not the same as a manual where you need to shift it yourself when things get a little harder.
A carburated 283 pushing a pickup through the air I'm inclined to agree the 3.08 might not work real well. But if you were talking a 388 stroker or something like that, I'd say go for it.
I have a 5000 pound four-wheel-drive pickup with a TBI 350, 700r4 and 3.42 gears that works pretty good empty. It does not pull a loaded trailer with the finesse I'd like but based on how it performs I'd expect a 3500 pound 1/2-ton to move pretty good with similar equipment. And many of the factory 1/2-tons have 305s with a 700r4.
A properly setup 700r4 is going to shift from OD to direct when it hits an incline and needs the gears. But running down the highway in OD if the motor runs smooth and clean you're going to get great mileage. Press the accelerator and you'll downshift.
