3.1L Overheating Intake Gasket - Head Gasket????

Short story........1997 Monte LS 3.1L, 105,000 miles. Driving to work, running fine but got about 5 miles from the house and the car just turned off, looked down and the temp gauge was pegged to the right. Did not boil over....

Took to local garage, paid $400 for new intake manifold gasket and my wife picked the car up yesterday, only drove 2 miles to the house.

This morning, started the car...idled kinda rough, got to the same place as before and temp gauge was pegged to the right again. Now the mechanic is telling me I should have both head gaskets replaced. What other possibilites are there? Why did he not check the car out....when he was done. What would you do? Thanks for any info! Much appreciated!

There are other issues besides head gaskets here, also it is a somewhat involved diagnosis to detect hd gasket problems, coolant in the cylinders, pressurized cooling system from a cold start, exhaust gasses in the coolant, find out how he determined this is the problem. If the engine was run till it got so hot it stopped, internal damage other than hd gaskests may have occurred. Scored cyl walls for instance. Was the intake leaking when he took apart, was coolant in the cylinders, lots of info missing.