Denise Price told me you cannot get 8 nights in a single reservation-max reservations are 7 nights. So you would need 2 reservations anyway.
I had the same problem (we are going for 8 nights) so I found a good deal on a 7 night reservation and then had to deal with where to stay the other night. There were no 3 BR Presidentials available (I guess it's going to be packed while I am there

) so we booked Wilderness Lodge with an AP rate for that one night ($235). Denise did say that if you have 2 reservations with the same type of unit, chances are they won't move you mid stay because they don't want to have to clean the unit.