rear Main Seal noobie question

Thanks for the help guys. I saw on one of the forms that the 360 magnum rear main seal also can work and it's a tighter fit? The person who posted it said he had a leak he just couldn't fix until he used the 360 seal. Anyone heard of this trick? I'm probably still going to go with the 340 seal but just in case that doesn't work...

I cant see how a 360 seal would work since 360 mains are a larger diameter than a 340.