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

I studded my 360LA which already were 3/8ths. and used some torque-spreading conical washers, like as are on the slanty manifolds.
if your stat house is flat and the receiving area is adequate, it won't take much torque to keep it from leaking. To make it even easier, I run a 7pound cap. The hoses and stuff last forever.
If you need a sealer, you can't beat "the right Stuff." but it's a PITA to remove later.