Boosted Falcon

hey, ive got a 95 EF falcon I recently had trouble with it overheating rapidly after 5 mins of driving it and also the water in the overflow keeps disappearing. I found the problem to be a large hole in two of my welsh plugs on the exhaust manifold side. i flushed the system and replaced the plugs with new 1's, started the car up and yet my car is still over heating, although it took around 10 mins for it to overheating rather then 5 mins which was happening before. However, i am not losing any water from my overflow tank anymore. I got a mate to look at it he took the rocket cover off n ect and the head seems to be in good condition theres, not cracks and signs of oil leaks what so ever, there is also no oil in my water or water in my oil. im thinking it could possibly be my radiator, thermostat or water pump, but my water pump seems to be working ok. would anyone know how I could possibly test the water pump, thermostat and radiator to possibly narrow down the problem? any advice or comments would b much appreciated.

Is it a big job replacing the welsh plugs? cos I think mine are gone too, they were leaking for a bit there and the car was overheating, flushed the system and the radiator, that didn't help. Threw some chemi weld in there and now she is running ok, but will need to get it looked a soon though.
sorry i couldnt actually help with your problem as I am having a similar one i guess lol.