It appears you are referring to the rust just inside the owners at the top rear trunk where the trunk lid closes.

That is part of the tail light panel. It is fairly easy to replace. You can try to salvage the one from your 68 or just buy new. They are inexpensive. $70 for a cheap repro or $120 for a good one
https://www.npdlink.com/store/products/panel_tail_light_improved_repro-151921-1.html

Not sure what you mean about the hood? It is bolted on not welded. Did you mean the roof?

 

Yeah the roof, i always get that messed up

It appears you are referring to the rust just inside the owners at the top rear trunk where the trunk lid closes.

That is part of the tail light panel. It is fairly easy to replace. You can try to salvage the one from your 68 or just buy new. They are inexpensive. $70 for a cheap repro or $120 for a good one
https://www.npdlink.com/store/products/panel_tail_light_improved_repro-151921-1.html

Not sure what you mean about the hood? It is bolted on not welded. Did you mean the roof?

 

Typically the roof frame is OK, just the outer skin is damaged. There a about 200 spot welds all along the edge you have to drill out then you can move the good skin to replace the bad one and rewelding all the spot welds. There are a number of pics online

 

I do not see a reason to replace the entire tail light panel just for that. You can replace just that corner piece like I did here. The pieces can be bought reproduction, and replacing it is probably only an hour job.

1967 Mustang Restoration: Botched quarter panel skin repair

 

what would that corner bracket piece be called? and also in the second picture near the windshield do you think that can be grinded/sanded down or does that have to be replaced too?

I do not see a reason to replace the entire tail light panel just for that. You can replace just that corner piece like I did here. The pieces can be bought reproduction, and replacing it is probably only an hour job.

1967 Mustang Restoration: Botched quarter panel skin repair

 

I don't know what you're referring to near the windshield. That piece is just called a trunk corner.

http://www.cjponyparts.com/trunk-rear-corner-cp-cv-rt-upper-1967-1968/p/M242R/

 

right there in the picture. do you think that corner piece will be long enough to reach the part where the rust hole is at?
Mustang Trunk Rear Corner Cp Convertible Rt Upper 1967-1968

I don't know what you're referring to near the windshield. That piece is just called a trunk corner.

Mustang Trunk Rear Corner Cp Convertible Rt Upper 1967-1968