If you don't like the way it performs now with the .70 OD the 700r4 will only make it better if it downshifts to accelerate or climb a hill. Otherwise you're looking at similar and apparently not inspiring performance.
Based on the way my factory stock truck with a 700r4 behaves I think that's a reasonable approach. If its not going to be acceptable for an automatic to downshift you better change those rear gears.
Provided the motor is running efficiently at your current cruising speed, I'm inclined to stick by my earlier suggestion of installing the trans and only changing the gears after seeing how it feels. You may still end up changing the gears, but if you like it without changing them you've saved time and trouble, not to mention a few dollars.
