greetings chris
the design water tank dimensions for the W28 are in the construction manual on page 6-D-2.
i measured the length of mine and it is 3" shorter than the design length. i can't vouch for other measurements because they are not accessible..
i assume you have read steve yoder's post titled, "those pesky W28 water tanks."
there have been other posts over the years, one that comes to mind was by a guy named eric. he posted pictures of the start of the process but nothing since.. biiiiiig job !
my tank leaks from holes in its upper surface, because of fasteners into and through the plywood that covers the tank. consequently i don't keep it fully filled.
the inside of my tank is probably not quite as bad as yours, but does have tiny flakes of white stuff floating around. i don't think the flakes are aluminum oxide, but rather some sort of aluminum salt. my tank is set up to collect rainwater off the deck, so it would certainly contain unknown contaminants. i patched a small leak on the lower edge with epoxy putty about eight years ago. my tank had a hole drilled in it low down, a little aft of the ice box. a nipple was fastened in place, so i can stick a hose on it and flush the thing out, getting some circulation between there and the fill pipe and visa versa..
now this may be a silly idea, but i have considered removing the tank by cutting a hole in the hull from the outside, rather than tear up the inside of the boat. the chunk of hull could be removed in one piece and then glassed back in place similar to a huge hull repair. before attempting this, i would certainly get some technical advice form some experts and proceed from there.
it would be nice if you could post some progress reports along the way.
boating is fun
james