Engine Block 2006 Scion Tc?

I blew my head gasket on my 2006 Scion Tc. Toyota says that my engine block also needs to be re-threaded and my service adviser said that he does not recommend re-threading the engine block because it I can be speeding down the freeway one day and the bolts may fail. He recommends that I but a new engine. What should I do? The car is no longer under warranty (145,000 mi).