Two things. First, be aware that drilling a 7/8" hole in the edge of a body is not a trivial task. Bodies are made of hardwoods, and the holes are often deep. So, make sure you have a good vise or something to hold the body firmly in place. Second, if you already have a smaller-than-ideal hole and want to expand it, you're better off measuring the holeyou have and gluing a dowel in its place, then drilling into that. Use a Forstner or brad point bit, and drill slowly to reduce/eliminate overheating.
All this is to say: if you can possibly help it, order the body with a 7/8" hole to start. You'll be glad you did. Why they even offer any other size is a mystery for the ages.