Agree with Hedgehog and the others... I don't think the 235 will be able to overcome the wind resistance at high speed in overdrive.
Remember you are basically pushing a shoebox through the air, and the wind resistance goes up dramatically. If you had a streamlined vehicle and a high-torque engine, it might work.
I'd leave the 3:90 in place and see how it drove with the OD transmission.
Once had a company truck, a base '82 Chevy S-10 with an Isuzu 4 cylinder. It ran fine unloaded, but if anything loaded in the bed was higher than the cab, it would actually run about three MPH higher in third gear than it would in high (fourth), due to wind resistance.
---> Bill
