I installed a set of the XT parelleogram v-brakes for a friend. I did not run into this issue.

Here is a thread with some good info (charts and pictures) on what brake boss spacing is needed for different rim widths. It is for one specific shimano v-brake, but I would think that they are all in the same ball park:

Cantilever post spacing question (xpost)- Mtbr.com