Alum water pump housing question

I installed a similar aluminum water pump housing on my 383. I recall it had a flat surface like that at the T-stat. I used an aftermarket stainless-steel T-stat housing which comes straight up (ebay). I recall it had a grooved recess to fit the T-stat, with the gasket below the T-stat as post 2 says. Avoid the chrome-plated T-stat housings. They look purty for a while, but the chrome flakes off to cause leaks.