Water keeps leaking in the head and bending valves?

Recently my water pump went on my Vectra 2.0 SRI 140, i purchased new stretch bolts, balves, water pump cambelt and all these fitted. The car started but as soon as i put water in the car wouldn't start and it was leaking from the head and bending valves.

We then thought it was a problem with the head so i purchased a second hand engine that was heard running fine in the car with know problems what so ever.

We then fitted the new engine in my vectra along with the new parts, same happened again started fine but as soon as water was put in, it leaked from the head and bent another valve.

Does anyone know what the problem is or what we might be doing wrong.

Any advise be much appreciated as been messing with car nearly 7 weeks and spending a fortune.

Thanks