Modding to Nikon requires you to remove the Leica mount and replace with the nikon mount. That's what I did with my F3...and got a Nikon to PL adapter too...BUT the problem is the Leica's internals are exposed a little too much on some of the lenses and susceptible to dust contamination. You don't want dust on the inside...take it from me it suuuuuuucks.
So yeah, modding Leica to Canon is much easier (and what I should have done!). You just pop the canon adapters (which are really cheap on Ebay) onto the Leicas, then just get a PL to Canon adapter.