D1trout , you have embarked on an ambitious project , I will be watching and learning . Just offering some thoughts , take them for what they are worth .
The aluminum tub has lots of appeal but it is not the end-all . It does corrode if not protected especially around a salt air or salt water environment . It is also has more heat transfer than wood . I don't believe any of the aluminum frame in an Avion is welded , all rivets .
If going with the aluminum tub I would not piece meal it . The top aluminum structure ends with a U channel at the bottom that is screwed to the wood tub . It could be detached at that point with some bracing to hold it's shape then re-attached to the new tub the same way . The new tub could be built completely ahead of time , with much more control .
Best of luck on the project and keep us updated .