As for sealing, ALWAYS check that surface by running it over some sandpaper on a dead-flat surface.
Usually the sealing surface is bowed.
IIRC, a thermostat housing for a traditional Chevy V8 will fit; at least the gasket does.
I always glue the gasket to the thermostat housing and either Permatex #2 or wheel-bearing grease at the pump neck side.
Also, test the thermostat in boiling water with a meat thermometer before installation.