If you keep the computer, any changes must be compatible. Does this car have a locking torque converter? Not really easy to change. I think your best bang for the buck, while remaining stock appearing are: 1-Freer flowing exhaust. Replace the pellet converter with a modern high flow converter. 2-Tune the ignition curve. 3-64cc heads and mild cam, like the old Comp 268HE or Crane 272. You could also step up to a Performer EGR manifold, it should fit. If you go this route, you might want to simply build a 350 or 383 and put your numbers matching engine in a plastic bag. 3-Shorty headers or Sanderson headers would help the exhaust, but only after replacing the converter. I think you want to look at increasing torque more than horsepower. 3.08 gears (if that's right) and a locking torque converter will be easy on the highway. I don't think you would get much benefit from an overdrive transmission by itself. If you can get to 300 horsepower and 380 lb-ft of torque, the car will be much more fun to drive. Try 1 and 2 and see what you think.