I would extend the new rafters up to the 4th purlin. You need something for roof sheathing to attach to anyway. Sister a bolster to the 4th purling where these new rafters mortice into, tusk or soffit. The bolster need not attach to the principal rafters but should be well fastened to the existing purlin and as full length as possible,tapering the ends with an adze.
Assembly? Stick the tusk tenon into the mortice and slid the foot along the plate aligning pockets to the now cut purling stubs, banging it home.
6x8 bolster? configured 8 inches deep into the living space, deeper than the 6 inches now.
