Answering my own post in case somebody is searching this topic in the future. I ended up using two o-rings under the Teflon ring to fill the gap and keep the Teflon centered on the piston. A dash 150 o-ring was part of the kit but as mentioned above this left a gap between the o-ring and Teflon which allowed the Teflon to move off center making it hard if not impossible to get the piston started back into the bore.

The o-ring I added was -041 which has a smaller cross section (.070) than the -150 (.103) I would have preferred to use two rings of the same cross section but this is the only combination of sizes I could come up with that would center the Teflon and not over compress the o-rings once the piston was in the bore.

Since I had to buy the -041 I bought a new -150 at the same time and got them both in Viton. I’m sure the Gates kit uses import o-rings made of who knows what so at least I know these two are decent quality.

The box was installed a few months ago and works great with zero leaks.