The only way I can see a connection between an overheating condition and replacement of the balancer on a GS300 is through the water pump drive belt. I believe the water pump has a separate belt with its own adjustment point and all belts must be loosened in order to replace a balancer. I'm betting the water pump belt is too loose to turn the pump fast enough at higher speeds or possibly any speed. Incidentally, temperature and pressure are directly proportional so as the engine heats up, the pressure will also rise. To address your concern with the cooling fan: at only two times will both fans run simultaneously. One is during overheat and the other is when the A/C is running. Check your belt..
Thank you, will have it checked. The mechanic told me that there is a lot of pressure in the motor that causes the water to backup in the reserve box. He suggests that it could be a blown head gasket, which I find hard to believe since the car never ran hot until the harmonic balancer was changed.