Holden Calais 1986 Problems

THE VL was powered by a Nissan straight six, not a V6. There is a problem with the cooling system of the Commodore, but it's caused by the installation of the engine, not the engine itself. Because of installation compromises in the Commodore, the Nissan engine is difficult to refill with coolant after being drained. If it isn't correctly refilled, the system can be low on coolant and the engine will overheat. A bleed valve on the top of the engine needs to be opened so coolant can flow out when the system is full. If it's refilled correctly, you won't have any problems.

I wouldn't recommend fitting a catalytic converter as I doubt you could get it working. There's a lot more to fitting a converter than installing it in the exhaust system. Making it work effectively is a development exercise in itself, and one I would suggest isn't worth attempting. At least you know that by running unleaded in your car you're not pumping lead into the atmosphere.