Someone posted earlier that the tree plan was ditched because the current hill wouldn't support it along with the boat stuck in the tree.
Except it wouldn't be a tree with a boat stuck in it.
It would likely be a steel frame with a fiberglass coated foam
(as in styrofoam) build which would all weigh considerably less than it looks.
I mean, weight could still be an issue, I guess but my point is, in a situation like this when they want to build something, the first step would be figuring out what and then the second would be the construction materials and methods for the circumstances they're building under - including weight load.
This also wouldn't likely be something just bolted onto the exterior of the ride but connected directly to the frame under the exterior.
Again, it could still be an issue
(one that money could absolutely fix) but I'm guessing that initial concept art they rushed out when they said this had been in the works since Walt's birth was probably the only solid thing they had and that little to no real thought about the actual story or practical efforts of converting this attraction had even been seriously considered at that point.
In short, I think it was about like the presentation of Beyond Thunder Mountain which is why it's taken years from that announcement to get to a point where they are ready to close the ride and actually begin a two year project that involves an already built structure and ride system.