Mine is a series 5 and I added the Razorback the old fashioned Kitfox way, i.e. a tube with plywood supports. Kitfox no longer makes them so I rolled my own. Went in easy enough. I used a forstener bit to cut lightening holes in the 1/4” ply support. For the tube, I used aluminum that was same dia as the fwd vertical fin tube. Made a bend jig by cutting a notch in an old 2x8, bent the tube to my liking and hysoled it in. On either end I cut a relief so it fit the mating tubes properly.
The only thing I wish I had done was to add the later elevator push rod and longer baggage area.

As for the aluminum tank, adel clamps are more than sufficient.
I have an aluminum tank that I bought from Kitfox and won’t be using... 🤔