Just guessing that your boats transom is set up similar to mine... I would think you would want to mount your riggers just forward of it on the side gunwales. Doing this will allow easier access to crank up a manual downrigger. Putting manuals too close to the back corner may make it difficult to use with one hand or the other. If they are Scotty manuals it may not matter as their cranks are on top... but others are on the side.
The only other thing I can think of that may help... If you plan to run planer boards/otter boats, mount a couple rod holders as far forward as possible, while still allowing you to see the rods from the drivers seat. This way your leadcore setups you have going out to the planer line are out of the way of fighting a fish in the rear of the boat. Mine are mounted about 4' forward of the riggers. Actually, I'm going to mount a couple rod holders to my rear hardtop support posts this winter to help move the lines forward even more.
Always nice to have extra room to maneuver a fish into the net. You never know what type of acrobatics they'll do when they are around other lines. Never fun when you have to untangle lines and/or replace leaders due to a lack of space.