The 4S5800 pack sits up against the fire wall, and the 5S5800 sits behind it to balance.
The Corsairs usually end up tail heavy, so you may want to hold off on a final pack(s) location till you have it pretty well built to save yourself some grief, or having to add weight. You going to add the working flaps? They are a bit of a bugger to get right but well worth it. You may need to do a little modding dependent on which brand you use. The retracts will fit and the wheel/strut should clear no problem with the wing.
Lot less to deal with if something goes wrong.
