Since they don't have Booking Categories, it's practically never too late. I'd call them tomorrow and have them note your reservation.
As far as where, it depends on you. The Springs and Grandstand are pretty close to main amenities of the resort. Congress Park can have a good DTD View and there is a walking path to DTD from there as well.
The Carousel and Paddock are more secluded and quiet. It really depends on what you're looking for in this trip.
Room requests are NOTED when you book, but NEVER guaranteed, even if you book 7 months in advance at SSR. You get what is available when you check in, however, you can ask what is available and pick your choice of available rooms or areas. Your room assignment is not given until you ACTUALLY check in..
MS will note your reservation with your preference, but the hotel doesn't receive your reservation until 5 days prior to arrival. So if you want to call SSR directly to make a polite request, don't try before that - they won't have your reservation.