Scot, I have a Series 5 plastic header tank that's taking up space in my garage. It has multiple connection bosses on the top surface, as others have described above. It had brass fittings installed by the original builder of my plane but has never had fuel in it. I removed the fittings and cleaned old sealant from the threads.

The tank is essentially worthless from a resale standpoint, so I would likely just end up throwing it away. If you find that re-routing your fuel lines doesn't help and you'd like to try my tank, I'd be happy to send it to you. Just PM me your address.