You need to get fitted properly to see what changes need to be made. You might need an adjustable comb - they cut the comb and put in adjustable gear and use the same comb so it matches.
You might also need an adjustable butt plate like the gracoil
http://www.graco-corp.com/t/gracoil
or the jones:
http://item.brownells.com/quaternary...ates/index.htm