Have you had the panel in the fuselage yet? You'll most likely have some interference with those radio racks and the F-402C bulkhead member.
A lot of RV-4 panels have cut into this bulkhead, but just so you're aware, it is cutting into a fuselage bulkhead structure member. I've also shown a picture of my instrument panel where the transponder on the lower left is as low as you can go without cutting into structure. The top of F-402C is level with the canopy rails.
My panel is also 1" shorter from the bottom edge than the plans show. In other words, on a plans sized panel, there would be 1" additional inch of panel space from the bottom edge of the panel to the lowest a long radio rack could be placed without cutting into the bulkhead. The transponder rack is literally sitting on the F-402C bulkhead in my airplane.