rethreading thermostat holes from 5/16 to 3/8 ?

I agree with @Alaskan_TA and others; do a Heli Coil. I have done a bunch of them, and they work great. In the time we have been discussing other options, you could have gone to a store, bought them, and had it installed. They work great and are easy to install. You won't be sorry.