Actually, your 1975 model is not so "old"
(or vintage that is). There are blueprints and floorplans available for the mid 70's models.
You can buy a service manual for that unit from Secretarial Services or you may keep your eyes on EBAY. I am almost certain that I seen a 1975 manual sell less than a few weeks ago.
You may be able to get someone on this forum to share their manual with you.
If you call Airstream, they could probably point you in the right direction.
Good luck with your restoration