I used the O ring that came with the new pump and it was just a bit too small. You would only notice it if you compared it to an AC Delco part. Luckily I had an Ac Delco pump here for my supercharged car and used it. FieroFool had an extra to replace that one.
I put in the new o ring and put 2 gallons of fuel in the tank and turned it upside down over a metal trash can overnight to make sure that O ring didn't leak.