Sill panel replacement tips

[poyda]

HX Sill join.jpgHi All
I have rares type rust repair sections for the lower sill and the dogleg in front of the rear wheel. My question is, how would you go about welding the seams back together where the sill meets the main qtr panel? Once I put the sill repair section up to the van, you can see that the new panel will actually hide the upper join. I suspect the outer shells here might have been spot welded before the inner structure was put in from the back. Any tips on how to weld these in properly without removing the backing panel and refitting it later? The back panel is pretty solid except for the little bit you see through in the photo LOL. Attachment 1470320160317_182810.jpg I was hoping to use a flange tool along the top just below the seam however the rust goes up pretty high so would need to cut off too close the seam to make a lip. Obviously I could just cut right on the seam and do a butt weld along the entire length if I have to. Anyone thoughts on how to do it right?
Pictures of the offending seam and the repair section enclosed....I haven't put a photo of the dogleg up but you know what I mean.
Poyda

[Vombil]

most just cut below the fold. i've heard you can drill out spot welds from the inner tub of the ute/van but that would be a nightmare. perhaps use a thin blade on the grinder and cut horizontally right in the sandwich

[poyda]

Thanks Vombil. More I think about it the less I think I can do anything else than what you suggest. I'll try to take care that none of these seams get filled in with weld or bog etc.