This is a pretty serious job you are considering, and there is alot to consider. After you remove the existing shingles and sheathing do your old rafters spring back? If so you are off to a good start. Just nailing a 2x6 to the existing rafter would only add weight without much structrual strength. The rafter jacks are critical there. Consult an engineer or architect to examine your existing roof to determine how to best repair it without adding too much weight. Then again removing the old shingles may be the answer.