Are all Mopar mid 60's - mid 70's wagons equipped for 3 rows of seats? In other words does a 6 pass wagon have the same rear floorpan/well as a 9 pass, just missing the seat setup?

No different rear sections.

The 6 passenger versions just have a hidden storage area in the rear where the 3rd seat would go.

What are we talking? Floorpan, or cargo floor? Floorpan is the same, 6 or 9, but, the cargo floor is different. On a 6 passenger, the rear floor is one big 'door' with one hinge. With a 9 passenger, The floor folds in two sections to form the back of the 3rd seat.

The floors will interchange, its a bolt in deal, but, you've got to change everything.

(I'm talking about '70's B-bodies)