Ya, I used up a handful of stainless nuts. As soon as one started binding a little, I chucked it and grabbed a new one.
There is no earthly reason to use stainless bolts or nut for this tool, and you discovered the best reason NOT to use them. SS nuts are much softer than carbon steel. Stick with carbon steel and use lube on the threads.
FWIW, I had ordered the other parts from McMaster, including the large SS coupling to use on the outside as a mandrel. I promptly lost it before the first use (fell out of my truck, it appears). I just took the reducing fitting to HD and picked up a 1" NPT black iron coupling. Works just fine, costs about $10 less.