Exhaust Leak - 318 Manifold

Ok, so I have been fighting a phantom exhaust leak at either my manifold to head gasket or the manifold flange to downpipe. I just ordered a set of remflex gaskets for the manifold to head gasket because I have heard great things about their products... my dilema is what can I do about the manifold to downpipe flange?
-Can I use a donut?
-Do they make a special flange seal for this?

The previous owner gobbed a bunch of copper high temp perma seal in there before, but I'd rather have something a little more durable than that. Any thoughts?

This is for my 70 Dart.... 318, stock exhaust manifolds, LA heads, dual pipes with one down pipe off each manifold if it helps.

Thanks,
Clark