Thermostat housing leaking

If you machine it flat, you may have to also machine the stat-receiver shelf. If left too shallow, the stat will cut the gasket, and prevent the sandwich from stacking flat; and that will cause the freshly machined housing to warp as soon as the bolts are tightened. Pressure test it before the blower goes back on,lol.
I install studs into the manifold, and use slanty manifold washers to spread the clamping load. Those special washers were also used on factory 318LAs on the 4 corner studs of the exhaust manifolds.
Up here in igloo-land I get away with a 7psi cap. That is an immense relief on the cooling system.