What heat range in an NGK plug is required for a 1979 Yamaha XS750 Special?

The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is determined by the heat range selected. The heat range has nothing to do with the amount of voltage going through the plug. Also plugs remove heat from the combustion chamber not add it.
1977-83 Yamahas used the NGK B-8ES or BP7ES gapped at about 0.6 - 0.7 millimetres or 0.024 - 0.028 inches .So your heat range equals a 7 or 8 eaqual a "colder" or cooler plug.
A cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature. A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads, or is run at a high rpm for a significant period of time. Colder spark plugs remove heat quicker, reducing the chance of pre-ignition/detonation.