There are holes in the trunk floor on both sides of the tank in my 66 Fastback.
Bummer

Which of these parts do I need and what is involved to do the repairs? Will there be any exterior painting needed due to cuttting and welding?
If the rust is confined to the floor, you'll need a trunk floor for each side. Properly done, the only visible repair will be on the pinchweld at the bottom of the quarter, which is blacked out.

 

Sorry, but be prepared for frame rail damage!

 

Blogged the repair of mine if you're interested. As stated above, frame rail damage is always a possibility. There will be damage to your paint on the tail light panel due to braces welded between floor and the tail light panel.

Multiple part blog series:

1968 Mustang Convertible Restoration: Kicking Trunk and Taking Names
1968 Mustang Convertible Restoration: Welding In the Passenger Trunk Floor
1968 Mustang Convertible Restoration: Replacing the Driver Trunk Floor
1968 Mustang Convertible Restoration: Finishing the Trunk

Parts Needed:

TRUNK FLOOR PANEL RH 65/70 MUSTANG
TRUNK FLOOR PANEL LH 65/70 MUSTANG

 

it was so hard. why dont you try to calculate the auto repair costs of the flooring. it will maximize your budget in case you did the repair.

What the hell are you talking about? This smells like and advertisement post to me.