Hollow bodied guitars have always frightened me in terms of building, binding, setting up and having all that surface area to flopping well sand.
Also I find they're a bit like BBW's - hard to get yer arms around & yer fingers where ya want 'em.
Having said that, I'm very impressed with your fastidious approach to tackling this "Gretsch" project. Should be a classic when it's completed.
I also know doodly about floating bridges (pontoons?) May I suggest you dudes look at 3M's double sided tape which is not spongy and is very strong . . . I use it on pinball targets &c with great success.