I use what I call “The Two Hour Test.” As an endurance MTB racer, if anything is uncomfortable within two hours of using it, “Houston We have a problem.” Straight out of the box, if it is uncomfortable, it gets returned. Period.
I ended up going with Lake for my MTB shoes because they specifically make a wide shoe for people like me who have Barney Rubble feet. Lake also has a basic measurement process which will have you trace around your foot on a piece of paper. For you, you will need a large piece of paper for the tracing. After I completed my trace and took the measurements, I was able to select the correct shoe. Out of the box they felt incredible. After two 12 hour MTB races, I can officially pronounce them as awesome. At a minimum, the tracing should point you in the right direction concerning which brands and shoes within those brands that should work for you.
Also, most companies skimp on their insoles. Make sure you look at the insoles and conduct some experimentation with different versions. This will require assessing your feet again to determine your arch and if you are pronator versus supinator.
Best of luck to you! Shoes, seats and helmets are extremely personal. Slow and smooth in the selection process should make you fast and comfortable on race day :).