I'm trying to put a standard Shimano v-brake on the fork of my touring bike that came with cantilever brakes. After installing the brake, the pads are so close together that there's not enough room for the wheel. The spherical washers on the pads are already in the increased clearance arrangement. Does this mean that it's impossible to use a v-brake on this bike, or are there other models of v-brake or brake pad that have more clearance?
I've wondered what the application would be for half worn pads but your situation seems to make sense.
Are the brake pads long enough that the fork legs are preventing the brake arms from opening fully?
If joejack951's suggestion still doesn't work, try the Koolstop Continental brake shoes. You should be able to reuse the spherical washers and the pad is small enough that the brake arms should open fully. Get the Salmon-colored ones if possible.
On old touring bikes with canti's, the distance between the posts is different enough from more modern bikes with canti's or v-brakes that the newer brakes just won't work. If that's the OP's situation, I'm afraid it won't matter what brake pads are used, the geometry of the brake setup is just too different for conventional, modern brakes. My '83 Centurion Pro Tour is an example of this-