Matt I agree I think there is a restriction between the water neck of your pump and the freeze out plug in your block. This is what worked for me, bolt your circulator flange to the water neck where the pump would bolt to, with a couple of bushings get to a 1 inch or 3/4 inch ball valve close it, put a garden hose in one of your radiator hoses off of your heads preferably the passenger side, plug the other one start to fill your block with water until the flow stops at normal house pressure, when it stops open the ball valve at the neck, stand back and catch the water so you can see what was in there. This worked for me. Just a thought.