As a “advanced size” fella, I will give you my thoughts. Its hard to find a rib vest that fits comfortably to the shape of the chest/belly/ribs area of varying size big fellas. Because of that, I recommend one that is soft sided and not the “shell”. Something like this:
I actually have an even older version that looks more like a regular rib vest, but has the soft plates that are not one hard shell. If I can find a picture, i will post it.
I use this vest as a 230 guy. It goes up to 50" i think. I got rib injury on very first race day, purchased this and was able to race through the 6 month healing process with this and have had zero issues in the 2 years since
The Bengio brand sells XXL and can verify it will work at least on the lower end of your heavyweights. We have a racer at the track that fits the description you have given and he is happy with a XXL. For me the fit is not ideal but offers protection. If these guys are willing to tinker I made the fit much better on mine by cutting the back pad in half down the middle and reconnecting. With an increase in width (about 1"), plus with the flexibility the back pad now has an ideal fit for me with a size XL for my 195 American unit.