97 Cobra help with thermostat housing leaks

My T-housing cracked at the bolt flange. So I bought an after market replacement which leaked so I ordered a Motorcraft part - still leaks even without any pressure applied.

An o-ring sits under the t-stat flange and gets compressed in one half of the housing by the rounded/cambered mating piece of the other housing half when bolted together. That means the t-stat is sealed but the mating piece is metal-on-metal?

Any help getting this one right is greatly appreciated (forget the part about why the housing cracked in the first place - chalk that up to modern mfg).

Likely answer

I had a choice since I apparently forgot what order the parts came off. Simple as it might be with only a few parts it didn't appear to be reasonable that the rounded mating piece doing the compression would sit on the o-ring (rather than the o-ring under the t-stat flange) but then as I brightly observed that makes a metal=to=metal fit (not good for liquids). The way it was described in another forum (if the o-ring stays centered while tightening) having it on top of the flange forms a seal between the two in that space.