Not alot - but a pucker job must be close to if not ?100 and it's something to get done right or you'll end up with rusty arches !!
You might get away without it, but with that offset it'll be mighty close. Imagine - the wheels will be sitting almost flush with the outer most lip of the arch kit, so any bumps and there will be contact with the rest of the lip bent under (Lame description I know - but hopefully u'll get what I mean)