The stock XS battery is kind of oddly sized - it's pretty tall compared to many MC batts - and has a smallish footprint, so unless you have a bigger space in your frame fabbed up for a bigger one to fit, or are going to mount it sideways, I'd say go with a battery spec'ed for XS650.
At 1st I thought the Mike's AGM was bad, but I've changed my opinion - kept charged, it's decent. It is stock size.
Or go with one of the Shorei's, a diff type which are smaller and lighter and seem to have a good reputation, but I guess are not 100% proven yet.
I bought one, and for now it's just sitting there, as my Mike's batt came back to life after a good charging and works fine.
If you search the forum, you'll find lots of threads re: batteries.
As far as searching this forum, I actually have better luck doing so on Google and just starting it with "XS650 forum" and then my search terms. For some reason that works better (for me) than using the search feature through the forum. Picks up TONS of stuff from this forum.
So you're doing the PMA mod and also a re-phase - how about the ignition? are you switching to a Pamco? or leaving stock points, etc?