Once the kit is installed (bolted in to the car) you can no longer access the spare tire area.
Some other conversion kits take advantage of the tire well space, by having some of the electronics or batteries down in that area.
As an alternative, I have also seen installations with gas lifts that allow a battery tray to flip up, so you can still access the tire.
Jerry Asher's "Spirit of DC" PHEV Prius uses lead-acid batteries in a flip up tray. I thought that was a nice design.