thermostat housing problem

There's nothing the matter with your thermostat housing. The Flowkooler 330-series thermostat they sold you will not fit a Slant-6 engine. Neither will any of their other so-called "high flow" thermostats. Those thermostats fit the V8 engines only. It does not matter which way up you try to install it, nor how much filing or fettling you do—it is the wrong thermostat and will not fit. If you manage to cram it in the cylinder head hole, it will be jammed shut by parts of the cylinder head casting. It's not a new or innovative thermostat, it's just a creatively-marketed very old thermostat design called "balanced sleeve". They are not superior or preferable to the other ("reverse poppet") design, but at triple the price of a premium reverse poppet thermostat, they're good for FlowKooler's cash register.

Use a Stant SuperStat № 45358 (180°) or 45359 (195°) after reading this to decide which temperature to use.