67 Dart cooling issues

How about air pockets in the back of the block. Try jacking up the front of the car and either let it set for a day or run it. If you fill it always fill a fresh block from the water neck with the t-stat removed. This eliminates air getting trapped in the back of the block that will cause steam pockets.

I also see you have a overflow bottle. If you see air bubbles in it while running then it could be a head gasket leak or a cracked head.