4.8L water pump issues

[DavidMcD]

New member, first post. I picked up a 2008 GMC Sierra1500 4x4 recently. Someone had installed a 4.8L with 243 heads and it keeps blowing the back off the water pump. I've had the radiator cleaned, put on new hoses and thermostat. All to no avail. It still blows the water punp. Sounds like blown head gasket or even warped heads. Does anyone have any ideas? Getting a little desperate. 

 

I got the truck cheap and now it looks like it was a deal too good to be true.

[davester]

Is it overheating?  How long between when the pump needs replacing?  Any sign of water in the oil or out the exhaust?