The "unbalanced" feeling won't be noticable if the tank is mounted inside the frame rail next to the driveshaft; panels and 'burbs came with one tank on the passenger side and don't (shouldn't) lean. Tearing out the spare carrier seems awfully labor-intensive and will put the weight behind the rear axle, which will change the handling slightly. Ideally, the mass should be centered, but if there's any load in the bed you won't notice the difference. If it's going to see trail duty, saddle tanks are better protected from punctures due to the central location.
Regardless of where the tank is mounted, you will still need to cut a hole somewhere for the filler neck. The cleanest approach I have seen is to fab a fuel door flush to the panel and build a sheetmetal "cone" to protect the pipe where it protrudes into the bed. Another option I've seen is to use a marine fuel filler and cap, which is a round flush-mount cap and mounting ring (it looked very slick on an AD 'burb). Do you want the fill pipe to protrude into the bed fore or aft of the wheel well? Depending on what you haul and how you load it, that could be a concern.
edit: Is this what you\'re looking for?
