Im going to joggle the outer panel where its going to join the rear quarter and panel then seam weld it, then panel bond the outer lip too the inner arch lip this will be filled then when clamped together it will squeeze out to seal the join to stop any water ingress in later life this will be cut off once set
Yes Huw its not an easy task needs a lot of thought before you start cutting out old metal unless you've done a few before , need to get outer panel sorted now
Epoxy primed all inside the inner arch and inner body of shell before Outer arch repair panel tack welded in place and bonded around arch and clamped once bonding has set i will weld the rest.