use a chainsaw, forreal
no seriously though, draw a line across both sides of the deck that's parallel with the back of the dropouts and is also however deep you want to cut the deck (say you want to cut off one inch, two inches, etc.)
then take a saw to that shittt
then, take your brake out and cut the slot the brake fits in back about the same amount that you cut off of the back and then take a drill with a bit that's the same width as your wheel axle and drill new wheel holes then put everything back together.
you should probably not cut it if you don't know exactly what you're doing.