Help Required - HK Monaro Rear Quarter Repair

Hi I am looking for some advice on the best/ easiest way to repair the very poorly repaired rear section of my Monaro Quarter panel
Looks like it was involved in an accident at some time in its life and the drivers side tail light section has been very poorly repaired,( Photos attached)
I have a donor section from a HK sedan but it seems that the sedan is actually wider than the monaro by aboutr 15 mm when measured from the boot seal edge to the edge of the panel
I had originally intended to just remove the damaged section and replace with the sedan piece i had until i just realised the sedan is actually wider accross this section
My thought is to try and narrow the donor piece I have so it is the same width of the monaro
If you look at the photo of the donor piece you should see how I have marked out a pie cut that i would take out and then weld back up at the correct width before then removing the bad section from the car and replacing it with the reworked donor piece.
If anyone has any better ideas please let me know as this seems a lot of work

Poorly repaired rear qtr


Donor Part with markup to narrow