I would use a 1/2" socket LOL. Not sure what the issue is, I don’t recall it being very hard to do. When you install the thermostat housing it helps to drill a #40 hole in the thermostat at 12:00 to let trapped air bleed. Then glue the new thermostat into the housing, and glue the gasket to the housing, using contact cement.
I drill them starting with #40 in the middle, then continue to step up to larger sizes until it gets to 1/4". Then you can usually use a 5/16 - 18 tap to cut the threads. Finish by vacuuming the shavings out.
Replacing inlet manifold. Torquing bolts to 15lbs on first pass - and behold - snapped a bolt. Time to ditch the 50 year old bolts and go and get a set of new fasteners