I've been doing a lot of mods, and doing them as I built the empennage (not just before attaching it). Conduit for whisker antenna (which is buried under rudder counterweight in VS) and rudder wiring. Rudder trim tab and servo. Modded battery box for Concorde 24V battery. ELT and strobe mounts. Oxygen bottle mounts. Flightline A/C. Pitch servo. Contactor and wiring for ground power receptacle (receptacle itself and oxygen fill under access plate in baggage compartment, close to door).
The point is that these things are all easier to do before the empennage is complete. It's difficult to get back in the tailcone once the aft top skin is on, and getting to the top of the VS once it and the tailcone are in place requires a ladder (and some 'special' words as the tool or part you need is inevitably not up the ladder with you). It's also handy, if you have them, to install the rudder cables before the aft top skin goes on, so you can more easily thread the cables at the rear and reach inside to position and secure the guides at the aft exit holes.
Your mod list may be (and probably is) different, but the principle is the same. What if you have an AHRS mount to install back there? However, there is a trap here ... maintainability. As you install things, try to think about how you will work on them later. Just because you can reach it now, don't forget that it may not be so accessible in operation. Worst for me are the elevator trim servos; Van's did not make it easy to get to them. The A/C system is also difficult, due to it's bulk. Most of the rest can be accessed fairly easily through the baggage bulkhead or externally (rudder strobe, rudder trim, and whisker antenna). But I learned that lesson the hard way because servicing some things on my RV-6A are a PITA.
The last thing is you may not know yet all the equipment you will be installing. Oh, well. Try to do the best you can with what you do know about and leave some room for the unknowns. I ended up putting my AHRS under the right passenger seat because there simply was no room for the Van's-supplied mount in the tail cone due to the larger battery and the pitch servo. Unfortunately, I had already put that area together before I came to this conclusion. Building and installing a mount and access panel was an exercise in flexibility and vocabulary but I managed, though it would have been much easier to do as I was installing the seat ribs.
