Been researching the subject myself, in preparation for my 36 chevy sedan. One article I read says that you have to raise the fenders an equal amount to the channel. Kind of makes sense, that way you won't be scrubbing the fenders with the tires.

Another article cautions that if you are over six foot tall, you might want to reconsider before chopping AND channeling, as you might not fit into the car too well after.

If you slum around the photo section on this site, you 'll see a couple of nice pictorials documenting channeling.