Yes, balsa varies widely in density. And balsa strength is highly linked to the density. In my own building each part is made from sheet stock that is appropriate in weight and strength for the job it does.

Although it is not good to rely on the balsa density for strength on something like a profile fuselage. I'd strongly recomend using carbon strips on each side or better yet useing side frames like the depron foamies do in order to use good design rather than rely on strong, and heavy, materials.

Google for "balsa density chart" and you'll get a bunch of hits for charts that can help you determine your sheet stock densities.