Here we go, the original housing has been giving me grief for the past few years, pitted and not true, so I ordered a replacement - original style from Ames.

Installed it and used a gasket and RTV (like I always have) and it's still seeping coolant.

I've installed hundreds of thermostats without issues - any tips or tricks for our cars?

Maybe just RTV and no gasket?

Thanks!

I had this problem on my trans am the outlet is pot metal or cheap metal and you can actually overtighen it and warp it or make it flex and it will leak try not making it so tight you will probably have to put new gasket in it now

After market chrome ones are bad. Stock pot metal ones bend and can be warped. Get a metal file and file the bottom to remove any warping issues. Put Form-A-Gasket or equivalent on the intake. Put it on the housing as well. Place gasket on intake. Snug it up but not fully tightened. Let it setup ~30 mins Then tighten it up some more. You don't need to have it REALLY tight as i just bends. Try that and if it leaks tighten the housing a bit more.
Important to get the right stuff. There are many types of form-a-gasket products out their for different citations. Some work for oil pans but not for water. I used a blue form-a-gasket product. You can do it without a gasket but you have to be careful to not to squeeze too much out when you tighten.

No gasket. Just use water pump silicone. I fought with gaskets for years. No more. The aluminum bows when you tighten it with a gasket.

...and just finger tight until the silicone cures then just snug it a little more. If you tighten it with wet silicone is will all squirt out.

Update: No gasket and Permatex RTV, finger tight, let it sit overnight then just snug it up - No more leaks.

For future info, we had the same issue with seeping coolant around the thermostat housing. I cured it by placing it on a block with sandpaper and sanding it flat. The intake was also seriously pitted, I used the same block and sandpaper to clean it up. It was so pitted that I was not able to take it down as smooth as the housing, but got most of the pitting out. This pic of the intake was "in process".
Cal

Yeah, both the intake (which was worse) and the original housing were pitted pretty bad. No gasket and a new housing fixed this one.
I'll keep the original housing as a paper weight