Electric fuel pumps traditionally pump better than they suck, meaning - locate the pump close as possible to tank. I believe that's why many OEM pumps are actually in the tank. I originally had my pump attached to the the floor, but moved it to where it is now. In front of the tank, below the top of the tank, near the rear panhard rod, similar to @Outsane 's location.
Good power connection on it's own circuit (not shared) would be my choice and as @Outsane mentioned, fuse is important. Not real excited about running pump with the ignition off. It's real easy to get distracted and not get back to the rig to shut it down, as necessary. As much as it seems like a good idea to pump fuel while the rig is parked, the negatives of what could happen when your attention is compromised, to me is not worth it. And for safety's sake, it makes sense for pump not to run with key off.
All joking aside, the LRA aux tank has been awesome. ~500 mile range on a 37's is pretty nice, and could probably chase a little further running a more moderate speed. Install was reasonably straight forward other than all the 'first time' quirks...could install the next one in half the time after working through the first install. Cutting the factory fill tube is simple, just measure correctly. And no wiring issues on my unit like @80t0ylc mentioned...the gauge in the cabin is accurate, on/off switch works well, and the auto-off feature is nice (when aux is empty, pump shuts off automatically). Once you've got everything mocked up on the truck, check your feed tubes between the tanks and the pump to ensure nothing is kinked or making tight turns...we fired up initially and pump was running but starving - found a little kink in one of the feed lines...easy fix (some of the visual access is tough once the tank is in place!). I recessed the crossmember just a bit deeper on my Labs rear bumper when fabbing with the intention of catching the rear mounting flange of the LRA tank, and luckily all measurements were close enough to drill some holes through and mount the tank to the bumper (rather than using the LRA provided crossmember...now in the pile of other garage leftover parts in case I ever switch up the rear bumper setup). The dual filler neck works fine, just be cautious once you're up around 23-24gal into the aux so you're not running the station pump at full throttle with the auto shutoff...had a little gasoline geyser out the side of the truck one one fill up when I wasn't paying attention. 
