I've got an aluminum-framed Specialized Tricross I purchased in 2007 when I weighed 260+ pounds. I've never had flex in the bb, but then again, I've never had much flex in that frame. I do have some old steel bikes - an '86 Trek, '83 Kuwahara, '78 Schwinn LeTour, which all flex, but not to the point it bothers me much. On those old friction shifters it's easy to trim the front derailleur as necessary. Those are all Reynolds 431 tubing, as I remember (maybe not the Schwinn). I love my tricross, but the old steel ride is hard to beat.
If you find a good old frame that works, please post it, I'd be interested to see what you end up with. Good Luck!