But then you're left with a foam top block instead of a balsa one (??).
Brett and I are telling you two versions of the same thing: extend the fuselage side up with something that you can easily make to have parallel sides, and build the rest to that. He wants you to use an extender (not a bad idea, IMHO), I want you build the top block itself from multiple pieces.
If I'm visualizing your plans right, doing your foam thing still leaves you with the problem of cutting an accurate slot in a balsa top block.