If you're going to modify the rafters anyway, just convert them
into the upper chord of a truss, using 2x3s as the lower chord,
and regular metal or plywood gussets. That will give you
all the joist-depth you can use. If you're converting a pre-existing
flat-ceiling into a cathedral, that means you're taking out the
existing ceiling joists and-or collar-ties, right? Have you
worked out what that's going to do to the tops of your walls,
in terms of resisting the outward thrust of the roof?

--Goedjn