The .56 will be fine, doesn't give as much vertical performance but will allow scale flight, and won't turn a scale diameter prop (14 inch) if that matters. Just don't get too heavy with the building. I'd go with a sideways mount with the .56. Inverted will make the glow plug difficult to access unless you also install a remote socket. Sideways will also keep the spraybar a tad higher than an inverted mount, more in line with the stock tank height, since you already have the floor in place.

I'm in favor of anything fabric to cover a Cub. I haven't used Super Coverite, but I have used 21st Century fabric and also Solartex. 21st looks great, but I couldn't get compound curves without wrinkles, and the fabric seemed to relax after a while. Solartex, though, is great, stays tight, easy to work around curves. I just recommend a clearcoat to fill the weave and make cleaning it easier.