However you feel it’s going to work best is probably the right answer. You’re there looking at it, and you seem to have a pretty good grasp of what to look for. There’s not going to be a big difference in 8 or 8.5 inch spacing on cob centers, so again go with what you think. You can always see how it does on this run and tune it for the next one.
You do bring a good thought, putting it into the middle. This bar is going to blow the other lights away in par levels, if your canopy is the most dense in the middle you may benefit more running it there. But don’t mistake lack of heat for less power. You’re just making light more efficiently therefore creating less heat.
I usually remote most of my drivers for that reason. Install it off to the side somewhere and run a couple of extra feet of wire. Depending on how you plan to hang the light, you can mount driver to the outside rail of the frame on its side. But it will throw balance off quite a bit, which will require a fairly rigid mount to offset the balance.