Mafia,
I am not sure about the 13x8's, as they may have a different offset to the 13x7 Superheavies I have on mine (45mm negative offset).
Anyway, draw a line following the original guard shape 10mm from the original lip (lip, not flare!). Draw another line 20mm from the lip. So now you have two parallel lines 10mm apart.
Cut along the 10mm line, and then fold a 10mm lip at the 20mm line. So in effect, you have made the aperture 40mm wider, 20mm on each side. You know have enough space for 13x7's at least, and also a new 10mm lip to affix your flares too.
I dont think you can go much wider than this, as the lower door hinge reinforcing plate will be cut into and this isnt a good idea.
For fixing the flip front I would use Dzus fasteners:
http://www.kenlowe.com.au/body_hardware.htm
http://www.hotfrog.com.au/companies/DZUS-Fasteners-Australia
Hope this helps (by the way, that's going to be mega-phat!!)
Cheers,
Tricky