I can't answer because I don't have any Rochester carburetors to look at, however I believe it should stop in the pump well prior to the complete compression of the pump return spring. About the return spring, you might post in the Parts Wanted section of the Swap Meet. I had a box of Rochester parts that I had kept since around 1965 but I sold them here a couple of years ago. Sorry.
I would be hesitant to use a thicker gauge spring since the two work together, but that's just me. My thinking is if more resistance appears on the bottom it will affect how the duration spring does its work. Jon H (CarbKing) can tell you more. As I recall he accepts phone calls on Monday and Tuesday.
