I'm only 6'3" but have a 38.5" cycling inseam and found much to my surprise that an XL (58.5cm) Giant Defy actually fits quite well. I did fit a Thompson 410mm seat post, mostly because I owned it already.

But did you check out the Flite 747 from KHS? Only comes in 2XL and 3XL AND has really long cranks too.