I have a collection of pieces from a '78 Monte Carlo that are similar to what you describe. 305, 350 turbo and 2.29 rear gears. This setup had 70 series series tires on 14 inch wheels and when I drove it, before taking it apart, it seemed to work reasonably well.
What tires are you running? Your effective ratio is taking tires into account may be so high that your motor is lugging going down the highway at 60. Find one of the ratio calculators and check it out.
You don't need an overdrive to make this truck race down the highway. If it won't do it now you either need more power or lower gears, not higher.
