The slapper bar is a thicker leaf that sits on the bottom of the leaf pack...basically so you can carry a little bit more weight, and it also sometimes helps with axle tramp.
Thanks to FAQ
To fit 15's you shouldnt have to flare the guards....i've fitted at least 2 sets of 15's on my car, and havent done anything except tighten the wheel nuts. It also helps if you have the right offset.