as you are saying maz the fan isnt sitting in the shroud properly because its a 6 cyl one which is shallow compared to v8..if it has a viscous hub and fan from the 6 thats the same as v8..it may be as simple as fitting the correct radiator shroud..
as for thermos there is really no need for them as v8 holdens of that model has never really had a problem of staying cool with factory correct stuff...your viscous hub may be old and worn so buying a new one would help it..same hub from hx to vt com in all 5.0 litres..still available from holden for around 125.bucks.dont buy an aftermarket one from ebay.