That's really embarrassing! I've flown with people who weigh the same as you and been sat next to them myself (and i weighed 260lb)... we were lucky that the flight was quite empty and so had plenty of room to spread out. At 260lb, the plane seatbelts barely fit me, if i did buckle them they were very uncomfortable and on some planes i had to pretend i had buckled them and just put my arm across my lap when they came round checking people. I should have asked for a belt extender but was too embarassed.