Bob is definitely right on the overbore, it can be a huge issue, especially if you have any core shift.

One thing that hasn't been mentioned is that it appears that some of the pump manufacturers are putting non-mopar impellers in their pumps. I saw one a friend had bought, (small block) and it had an impeller that was between the diameter of the small and large Mopar ones, with 8 blades. I think it was a GM impeller. I don't think anyone knows if the flow and pressure out of a pump like this is correct for a Mopar, or what speed it should really run.