It depends on what you have, TT or XT.
The TT500E (1978 model) was the last year of straight axle forks, with the TT500F ('79 Model) having leading axle forks. The XT500's were a year behind with the change, with the XT500F still having straight axles forks and leading axle forks starting with the XT500G ('80 model). The fuel tank changes (and therefore the mounting point change) coinsided with the fork changes as pointed out by GMC. All the leading axle models (TT and XT) used alloy tanks. With the straight axle forks, alloy tanks were only fitted to the TT500's (76 - 78) while the XT500's (76 - 79) had a steel tank.
CJ