Use the 700PP tune at Exeter Gt tuning garage. It is listed in the VGT car section of tunes.I haven't found any tune for this event for LM55 :/
Lower power to 50% and add 57kg ballast to reach 600PP.
Depending on driver skill, the car is capable of beating the TS030 which has always been the fastest AI car in the races I have done.
You really should buy the TS030 and tune it. You can also use this car in the TT event. Should be easy gold in both events.