Never get into a situation where you are returning a vehicle you have bought to a used car dealer for "repair". Even if they "fix" it, they will do the absolute cheapest job possible since every dime they spend on the repair is lost profit. The worst thing you can have is a botched head gasket replacement - that's worse than just letting them leak and dealing with the smell, etc. We have seen them blow out and overheat, we have seen them strip the heads on the block, use the wrong parts, put the head bolts in the wrong holes, etc, etc. It's a bad situation to be in. Sounds like they did a crap job with crap parts.
GD