Thermostat housing on sbc 400

[Malamute]

The PO had a cheapo chrome housing which started leaking recently. Pulled it off today and there was a lot of grimy stuff in there and in the threads of the manifold. There also was just rtv and no gasket.

Two questions: Should I use a gasket w/ rtv on each side(what I always thought you should do) and what is the best way to clean the threads out? I don't want to get the bolt cross threaded in the manifold, that would not be good.

Thanks.