Hey mate I have a set of streeter rear guards on my car, we basically just cut out hte majority of the old guard, folded it up, welded it and it some stuff thats a bit like sound deadener or something... cannot quite remember. The basic idea is to just seal it all up.
Once that was done the guards were just lined up and put on with some decent sized screws (sounds dodgy but it works). Alternatively I think you can put in some bits into the frame that you can bolt into.
As for rims, I just have a set of 14x6.5 +15's on the back but I run 25mm spacers which works really well with these guards. maybe something to think about if you only have small wheels for the time being.