There's not too many upgrades that keep a stock look. Some are automatic. For example, most replacement alternators are rated at 42amps while the original one was a 38amp so you get an upgrade without knowing by replacing it. One big upgrade is to have your radiator rebuilt using a three row core vs the original two row. It is not a visible change but it is a good one. Keep this in mind, your car is 44 plus years old and its still operational. Replacing a lot of the parts (hoses, belts, etc) with new, rebuilding the major components (front suspension, heater) and going to radial tires will keep that car going even longer than the previous 44 years because you made up your mind to do something positive.