Make sure your thermostat has a small hole drilled in it to let air bubbles through.
Also - if you haven't flushed the block - you can get all kinds of crap out of the rear block drain. I have gotten as much as a coffee can full of rust, sand, casting core wire segments out of old inline 235's 261's when flushing them out with the head off and using a wire hanger to plunge down through the coolant passages.
