Well I spent a heap of time on Saturday experimenting with different configurations (thermostat out, then back in), checking and verifying carburettor operations, and substituting my radiators for the larger ones. The engine was still overheating with all the small changes I made, one at a time, so I changed the radiators which solved the problem!! No more overheating with water temps steady in cruise at 65*C and getting close to 70*C on WOT climb.
The carbies both checked out for correct jet sizes and perfect operation, but still only 6200RPM maximum. I pulled the spark plugs and they all have a nice tan/brown colour tending on the light side, so I reckon the engine is getting enough fuel. I'm going to have to live with it I think, which won't be a problem given performance for my purposes is more than satisfactory.
Thank you all for your help here, it's been very helpful to me.
Pud
