Outer Limits 4x4 Board

need to trim the back outa my rear arches and dont want to quarter chop at this stage (will when theyre smashd and ugly)
wat is the best way to do it? i was thinking of marking out where i want it to end, then measure about 20mm back, cuttint there then folding the 20mm over for the inside lip. sound alrite?
and how do i rejoin the inner seam back to the outer skin?
if uv got any pics id love ta see em
cheers
waandy

you would want less than 20mm lip, and rejoin is by welding plate from the inner to the outer, but you will need to repaint.