I am 6' 280 ish lbs. Bibs are far superior to shorts IMHO and will only buy bibs in the future. I personally have had a lot of luck with XXL Louis Garneau bibs and shorts.
I bought these bibs from Performance in a XXL and they fit great and were on sale at the time
http://www.performancebike.com/bikes...32_-1___000000. Looks like they are available in a XXXL too.