When I installed the original pump the impeller was tight on the shaft so it had to have loosened sometime after the first time I pulled it out and installed the anticavitation plate, because it was still tight then as well. Keep in mind this car probably only had 250 miles on it from the time when the engine was installed and now.
AS to the different pumps - I looked at the stock and milodon pumps, they had the same impeller and distance from the pump body that i could see using my eye. I did not notice whether the impeller was tack welded on to the shaft on either of them.
Maybe a good lesson would be to add a tack weld to the impeller and the shaft ?
As to the heater trick - the car has a heater and I did use that at different times to attempt adding extra cooling to the equation....
Anyway - its fixed and I expect the car to get a lot more miles on it now !!!