Call the Disney reservations line, or add a request through the chat feature on the WDW website -- ideally, with a couple of specific room numbers to offer as an example. (The touringplans.com room finder tool is handy for this.)
I wouldn't be too worried about it, though. Even if you're in the furthest possible room at BC rather than the closest, so long as you take the most direct route when you walk from that room to EPCOT, you're adding 5 minutes to your walk at best.