Here's a little Mopar World secret for you ...
if the thermostat housing is Chrome.. it will not seal.
You'll have to surface all the chrome off the mating surface, 'scratch finish' and try again. Two times in my life I bought a chrome thermostat housing once when I first started out doing this...it leaked , I removed it. The other time about 15 years later when I forgot about the first time, it leaked and so I put an old rusted 1968 housing on after a wire wheeling...never leaked a drop.lol
Skip the fancy thermo housings and just clean up a stock one and paint it aluminum.