Installed the racks tonight... was able to fit a 48"x18"x72" rack and a 36"x18"x72" rack together into an "L" shape in the back corner. I put a much smaller rack directly under the AC unit just for a little more shelving. There is a lot of deadspace in there but there is also a WHOLE lot of shelf room too.
Couple of photos of the shelving:
And one of the little shelf:
