I wouldn't be shopping for suits at Marshalls. From what I've seen when I've been there, its pretty garbage.
Don't buy a polyester suit. They don't breathe, they don't drape properly. Sometimes commissioned people at a store with more expensive suits can be helpful, so that you know what to look for in terms of fit.
Or creep the suit critiques on this site religiously.
40s does not mean slim by the way, it means short.
Do yourself a favour, and go try on a ZZenga suit...