Mitch-
We have been running UBBT 6.x with Photopost for a few years and the integration is pretty solid.
As for your scenario, you have a large number of photos in photopost that are currently attached to PP users (and subsequent user id's). For you to move to an integrated login for both PP and UBB you will have some challenges with the historical data in regards to user id's.
So as gizmo said, your first step would be to upgrade to UBB 7. After that you will need to make a 'map' of old PhotoPost user id's->New Threads User Id's and update your historic data accordingly.
It will be a bit of work, but in the end you'll have the integrated login!
