For reliability, I wouldn't suggest a cam or carb upgrade. To get your car running they way it should all you need is a decent tune up. It will be your cheapest option. A clean out of your carby would help a lot and there isn't a lot of mechanical knowledge needed to do this. After giving your carb/fuel pump and lines a clean up, id advise you (if you havent already) to throw some money at your electrics. Spark plugs, plug leads, coil could all be replaced. Rotor button and points also. Checking the timing wouldnt go astray either. Best of luck, and i must add, thats one good looking sunny.