Hi John,
Hope all is well up North.. The last time I did this I ended up cutting a few inches beyond the panel that I needed. Once cut out I removed the section by removing the spot welds. PIA I know. The hardest part is being able to cut away the section without damaging it. Mark and cut a little at a time. Guessing this maybe metal for the 6?
Now on my 6 I ended up drilling out the spot welds for the sections I wanted to remove. I will warn you not to do it until its braced and on a chassis jig. I have my 6 on a bench with fixtures. It's pretty solid when its all bolted down but still need to measure before welding it up. I had a hard time with a door post that kept moving ever so slightly. Threw my door gaps way off.
Check the link for body sections:
http://p914-6info.net/PDFs/Group%208%20Body.pdf