Here's my idea. Use one side of the GH for shelving and go to Lowe's/Home Depot and purchase their enamel-coated, wire shelving that is 8' long and 18" or 24" wide. They will have all the hardware to put it together and you can have two shelves, the first shelf being 2' off the ground and the second one 4' off the ground. That will give you room to hang that fixture and you can position the fixture a couple of inches from the ceiling. Those two shelves will give you 240-320 sf. of pot space, plus the space on the floor, below that first shelf.
Here are some pictures of those (Lowe's ) shelves that I used in one of my 10'x12' greenhouses. They are 10'x2' and on both sides (I have more room).
