Viewed your pictures, looks like you have quite a project ahead of you, and somewhat of a game plan. The only suggestion or thought I would have is this. Airstream may not have had the best designed roof for the prevention of leaks on these early models, but I do believe the whole coach was structurally designed as a complete unit, and engineered to be safe & sound. Its true, what you have now with the rusted out cross framing supports is bad, I don't think AS should or can be blamed for poor building practices when a roof is in the condition yours is in. There has clearly been an owner neglect issue leaving you with a major project to fix. What I'm trying to say here is this, in my opinion unless your friend is a structural engineer, I would be trying to rebuild the new roof as close as possible to original, any deviation from the original design might structurally affect the integrity of the coach. I'm not talking about changing from a fiberglas roof to a rubber roof (though this could be a factor of strength in design also) but I do question changing the design of the roof & how it might affect the rest of the coach.