Ya, DEFINITELY hold out for an XL, XXL or XXXL frame..... (depends on the maker but you'll get the gist that so & so's XL is the other brand's XXL, etc. etc.).

I didn't know that about Bill Walton, that's pretty cool. (though at 7ft, I can only imagine custom would certainly make sense)

Here's another thought......maybe get hold of the local rep for the brand(s) you like. (Cannondale, Specialized, Giant, Trek, etc.) Explain your situation to them and see if they can help. Getting closer to the source can often net results that the distributor might not even be empowered to.