I have a 125 RR tank that is about 3" from the wall. That's plenty of room to drop a ph power cord, etc down the back side so I can connect it to power under the tank. You should replace the outlets in that wall with GFCI outlets.
In 5 years I have never had a problem where I needed more room to the back of the tank. I occasionaly have needed to grab something that fell behind it from the sides though
Thinner arms would have helped
One thing you will need to consider is that the bulbs will need to be changed every so often, so access to the light fixture is important. This is especially true if you get a 6' long light fixture instead of individual pendants.
You wil want to make sure that you plenty of room over the top to get to the rear overflows. They do need occasional maintenance (running a brush through to clean out any gunk). You just need to be able to access them as most of the work will be under the tank, so don't completey fill that area.