trunk area rust issue

[tvgz28]

I have 78 280z and have bit of a rust issue regrading the area from where the trunk floor steps up right behind the seats to a little behind my strut towers. Im cutting out the spare tire area to mount a fuel cell soon and i want to take care of the rust issue but im not sure what is the best way to tackle it.

 

My first thoughts were to complete remove the entire area from the step up at the seat to the tail light panel and just tube frame it and throw a bead rolled panel back in but im pretty sure the rear sub frame is spot welded to the panel, Then just doing a good rust treatment also crossed my mind but i would rather just cut it out if its not going to be a nightmare.

 

any insight would be appreciated!