Dress shirts -- go with fitted.
Best advice on an off the rack suit is find "athletic" fit because they sell the pant sizes significantly smaller (e.g. if you buy a size 50 coat, they'll come with like a 46 inch waist pant; athletic will actually be closer to what you need). Either way, you'll almost certainly need it tailored. If you're doing anything with real size in your arms/shoulders/back/chest, that's just what's going to happen.
I've cut down from 260 to 230 over the past couple months and may be smaller, but am happy to have a much larger range of access of my closet lol.