The PA-14 top deck is really worth it. I flew my -12 with a huge baggage, but the original cross brace, and it was always in the way. When I rebuilt in 96', put in the -14 top deck and it is really nice. You can easily load over the top of the rear seat, or in my case load the rear baggage from the outside access door much easier with it. For just a little welding and a field approval when you are in the middle of a rebuild process, GO for it.
We built a trick little tray to hold a lightweight battery up in the boot cowl with an outside access panel. This makes a short cable run to the starter, keeps the battery outside of the cabin, and keeps it under the engine cover for winter preheating. We also did a underseat location, but don't need two batteries, so use the underseat area for storage of tool kit, and keep primary battery up front.
Hope that helps, there are photos of my -12 in the user gallery.