One of the guys I hunt with occasionally is taller than you but probably the same weight. He uses one of the Cabela's bags designed for big guys and seems to like it. I'm not sure which model it is (I know it's rectangular) or what it's rated to but I know he's had it for at least a few years.
Personally, if bag weight is not a concern i.e. truck camping, I prefer a rectangular bag so I can move around, put clothes and water bottles in the foot, etc. Despite being under six feet I always buy the long version of whatever bag so I can at least put a water bottle in the foot.
ETA, Not sure what type of foam mattress you have but a closed cell foam is warmer than an open cell and both are warmer than an air mattress. A good pad can make a huge world of difference in your comfort.