With all the work you plan on doing, I would look at this like simple template swiping.
In other words, all you are doing is taking the "structure" and using it as your own.
I not only approve of this, but think it's a great way to get a rythm going. A lot of top copywriters don't talk much about this, but to me rythm is right up there when it comes to writing great copy.
The copy in itself starts as a steady brook, gently rolls over small stones, then builds to a crescendo, spilling over into a great waterfall in the call to action portion.
When you take a really, really good sales letter and rewrite it, leaving just the structure (or skeleton) intact, you've got a great foundation for some good copy.
As long as you're not taking large portions out of non-MRR (master resale rights) material and using it as your own, I say "go for it"!
