Tress,
I installed the QAII last year after getting a deal from a guy who was selling the compressor, 2.5 gallon tank and all the mounting hoses & couplers for $180.00 new in the box. He even delivered it to me! I was looking for a OBA option at the time, and while there are better solutions out there...for the money it works for me.
I am not sure of the form-factor of the vehicle that you are installing it in so my info will only relate to a Jeep Wrangler install.
I did a lot of research on the "best" location and mounting options. I have some very specific requirements/problems.....I have a 99 TJ and require the use of my back seat (kids) and my limited rear storage area has a top access Tuffy Box. This takes up the rear area of my Jeep.....under the hood was the next spot that I looked at but on the right side I have a snorkel and dual battery tray that takes up most of the free space. On the left (drivers side) in the ideal spot I have a dedicated ARB compressor for my lockers. Also my research showed that the QAII does not like the high heat engine compartment.
Well, there is a "dead space" located to the rear of my roll bar in the back between the tub and roll bar that I decided to use as the mounting location. Next thing was a location for the 2.5 gallon air tank. All of my space issues remained and I decided to mount it underneath the floorboard on the drivers side beneath the rear passengers feet. (This is opposite the muffler). My Jeep sits on 35" tires and 6" Long Arm SJ Rock Ready Lift so I have some clearance.
I fabbed a mounting bracket and welded it to the roll bar for the compressor and drilled a hole and ran the tubing and wires down and along my frame. The tubing runs over to the 2.5 gallon tank and the wires to my SPod & Source for power. From the tank I ran air lines to couplers to the front and rear of the Jeep. On the front I fabbed a mounting bracket and it sits next to my stinger bar and at the rear I drilled a hole in the tub and ran the hose straight down and to the tank. I also have an pressure gauge in the cab in the SPod that lets me know what the pressure is and an electric shutoff once the tank gets to 120lbs of pressure.
At the end of the day, I like everything about the install and would not change anything. I do some pretty serious wheeling in it and have not had any issues with the tank under the floorboard.
Here are some pic of the install. Let me know if you want to see anything else.
Pardon the tape, it was to cover the ports until I connected the fittings...it is dirty under there!