Howdy and WELCOME to the forum!
As with most wagons from back then, the spare tire well should be in the rh quarter panel, behind the trim panels under the rh rr quarter glass. The space under the rear load floor on 6-pass wagons was usually an out-of-sight storage location.
Adding a third seat is not generally advised. Reason? First thing would be the necessity for rear seat belt mounting locations in the floorpan. Second thing could be that the 6-pass rear floor panels and 9-passenger panels would not be the same, which would mean fabrication and such needed to do your conversion. Not to say it can't be done, but it could well end up being much more than expected to do.
Now, I could be wrong! Perhaps they did use the same stamped panel for both 6 and 9 passenger rear body areas? If you dig around and find welded-in nuts for the seat belts and other welded-in nuts for the seat mounting, you could be in luck.
www.mymopar.com has free downloads of Chrysler parts and service manuals. They might not have the particular model year manual you might desire, for your particular vehicle, but for body, chassis, and some mechanical items, as long as the manual is for the platform of your vehicle, they might be helpful. Also check the links to the Chrysler MasterTech videos.
Keep us posted,
CBODY67
