In spite of what some are saying, I think adding some redundancy is a great idea. I agree that a buddy is a great option, but being able to sort out your own problems is the best solution by far.
There is no excuse to run out of air, and the odds of a serious regulator malfunction are minimal, but both still happen. The ONLY downside of slinging a 40 is cost, so if that isn't a factor, then go for it.
In terms of tips, the standard stage hit that you would get from Halcyon or Dive Rite is simple. Pay attention to the sizes of the bolt snaps and be sure that you can manage them with gloves. We mostly wear dry gloves here, so large bolt snaps are helpful.
I didn't notice if you wear a BP/w or a BCD, but obviously you will need a suitable attachment point on both your shoulder strap as well as your waist.