If you'll peruse the Clydesdale forum, you'll find quite a few folks that are over 274 lbs that ride fairly conventional bikes. The weak point in a lot of bikes seems to be the wheels, not the frame itself, but there seem to be quite a few wheels that will work, too.
As for building a Big Dummy up from scratch- I don't have a clue how to go about doing it, but if I wanted one, I'd just get down to my local bike shop, tell them generally what I wanted, and I'll bet they could fix me right up. There's no law that says you have to personally decide on each and every component.
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"be careful this rando stuff is addictive and dan's the 'pusher'."