Is it a raw fuel smell or from rich exhaust?
If raw fuel but you see no obvious leaks, check the gas cap & any vapor recovery systems parts (I don't know what a '76 Euro model might or might not have).
If it is due to excessively rich mixture leading to unburned fuel out the exhaust, adjust the carbs, check for exhaust leaks and try a down turn tip on the exhaust outlet. If the exhaust outlet is too short the shape of the back of the car tends to draw fumes into the car. A longer outlet or down turn tip directs exhaust out of the negative pressure area behind the car.