Surly is aware of the existence of XXL riders, but they're into making money as well as bikes. MInimum quantities, you know. Why not simply an XXL Pugsley? The XL Pugsley will allow you the kind of fitting a cyclo-cross bike, with appropriate handlebars. Don't expect MTB-performance on all accounts from it though, and at 300lb you may be pushing it on any Surly anyway.
Don't wait, just order a custom snow/sandbike.
I 6' 2" and have an XL Pugsley and its a bit big for me. With the right stem and seat post I think it could work fine. With 36 spoke large marge DH rims and the 4" endomorphs I don't think you'd have any problems. Surly frames are pretty tough. I haven't seen many people complaining about broken ones. In fact, now that I think about it, I don't know if I've ever seen one broken from riding.
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