With the hundreds to thousands of holes created by the interior pop rivets dead airspace "seams" unlikely, fiberglass insulation does settle and compact over time even if glued in place. Some areas of my 66 were fully intact, some completely uninsulated. Attempts to re-insulate will certainly be labor intensive. While I'm no insulation expert, My dad buckled the roof of a station wagon in the 80s with spray foam. Foams expand and create heat as a byproduct of curing, both of which can damage your interior and exterior skins.
If you are adventurous enough to blaze a new path with foam also be aware that there are many compartments created within the walls by ribs and braces so you would need to ID the sizes of each (trace rivet lines) to not over foam. Also you will encapsulate all wiring in foam with this approach. Don't get me wrong, I want to see this work. It could save hours, frustration, and BTU's over the conventional approach. Just food for thought