the jefe grande would be a great choice, the older version of it was called the "Gus" and its a great boat too.
what you really need to do is get some instruction on whitewater boating and try out different boats. if you dig it, then make the plunge as its more than just buying a boat as you need special gear too. also, for really big dudes looking at inflatable kayaks is a great option, particularly if you don't want to go through the learning curve of whitewater boating.
also, most whitewater boats cap out at a weight of 285lbs. there are a few, not many, that could go above that, but beyond that weight level there aren't many options. however, an inflatable kayak can go up higher than that.
shoot me an email via this website if you have any more questions.