Compound snips, such as Wiss, will be easier on your hand. Most hardware stores sell them these days, and I think I've seen them at Lowe's. They make three. One is for straight cuts, one for left turns, and one for right turns.
If you cut with snips, don't let the blades close completely in the middle of the cut. It will leave a mark, or maybe a small crack, in the metal. In T3 (heat treated) condition, this metal is more brittle than ordinary aluminum--and almost as strong as steel.
Use a file to take the sharp edges off newly cut metal, even the edges left where the supplier cuts it with a metal shear. A few minutes just running a flat file down each edge will save a box of Band Aids.
Please post pictures as you go. We're doing the exact same conversion on our Safari.