You really only want one or two cd's worth of a gap just so the tub slides in and out easily. You will be surprised how flimsy those 41qt tubs are right in the middle. Once they figure out they can push in that area you are doomed.

As far as the heat tape you could always take the back off and run it that way instead of taking each shelf out. I am not sure if the paint will be harmful or not other that that it looks pretty sweet.