Someone else may be able to help more, but I've changed the roof on a datsun and the joins were right at the top, which is apparently common on most cars. What I would do, is find a donor and sand the tops of the pillars back to bare metal to find the joins. You can cut them at this point and do the same on the other car. You can't get it too wrong because you are only change the roof skin, not the roof itself.
'Bout right.
