That is a pretty minimal adjustment. If you are worried about stiffness/sag, and figure strength will follow along, then devide the 340 by 275, and take the cube root of the result. That is the percent increase in overall size, with the same proportional wall thickness, you would require, very roughly. Of course that exact tube may not exist so use reasonable judgement. also keep in mind that your weight only affects a relatively short proportion of the beam. Possibly just stiffening up some brackets or seat elements would be enough