[Tx Firefighter]

Swapping a 700r4 or a 5 speed is going backwards. Your main complaint is that the 3.08 gears won't allow the engine to pull hills without downshifting. Adding an overdrive on top of that with your trans swap will just further aggravate the frequent lugging and shifting issue.

A turbo 350 would be a better trans choice. It would drive just like it does now down the road, but when it needed to downshift the trans would do it rather than you having to manually do it.

I don't know the type of hills you're dealing with, but I strongly suspect your engine needs tuning or something similar. It sounds like you're down on power from where you should be. An unloaded swb truck with a 230 and 3.08 gears ought to motor up most any hill at highway speed without too much difficulty or down shifting needed.