Here's whats easier to do with the aft upper skin off the airplane:
Mount the VS and HS
Mount, and set initial rigging for rudder and elevators
run wiring for tail light and pitch trim servo
Fabricate mounts for ELT and any remote mount avionics you plan on installing aft of cockpit.
Install conduit, studs, or cable ties fixtures to secure wire bundle runs in aft fuselage.
Install aft battery location and run cables
install Txpdr and ADS-B antennas
Yeah, you're probably gonna "uninstall" a lot of this stuff to get the aft upper skin riveted on. But, it'll be a lot easier to reinstall it than it will be to do all the locating drilling and fabricating after the upper skin is on.