I currently have a 3.42 opern carrier and want a 3.42 posi. Can I just buy a 3 series posi carrier (with 26 spline, thats what I have now) bolt my ring gear on it and swap them? If so what steps to change them, or is it not a DIY job? Thanks.

as long as the new carrier matches your housing and gear ratio you can just swap them and reset your lash. this can be accomplished by the side shims only if you dont remove your pinion.