Good to see you have got started on the build, glad the title came through.
When you tackle the inner fender and battery box you will need one of the reinforcements in the picture. The inner fender does not come with it and the Mustang supply houses do not have them. Somebody on Ebay makes them and sells. They are sold out quick each time they show up. I got one when I first saw them couple years ago. The part actually had the weld nuts that the actual battery tray bolts to. You need the reinforcement before you start it goes under the inner fender and sits on the frame rail horn.
I see the car is black. Does the car have the door sticker? If so and if it came from factory as black it will have a 6 digit DSO code and no paint code on the sticker. First two numbers are the actual DSO code and last four are the Special Order number. Ford has done this since they launched the Mustang. The first one was a coupe with real leather interior for Henry Ford II.
I have a 73 factory black vert. They only made 38 Special Paint fastbacks with standard vinyl black interiors AA code for interior. So it would be a rare find but rare does not make them worth more in some cases. So if it was Special Order for Paint & Trim it would have came with an Argent front bumper and the rubber fillers in front of the rear bumper will also be Argent. Yours probably painted like mine.
The reason they took black off the list was due to the horrible condition of the bodies going to paint. It took lots more work to get the black cars ready to paint so Ford took off the option list in 72.
If you pull the seats out and take the carpet up look for your Build Sheet. If Special Order the Special instructions will be on the bottom half of the Build Sheet in the Remarks section. It will state to install the Argent Front Bumper for sure.
If you order a Marti he can only confirm that it was a Special Order for Paint & Trim but does not have the records of what it included. It was shown as an option on the window sticker and cost $99.95 on my vert.
Picture of reinforcement you need, copy of my Marti for black vert, picture of door sticker, picture of the build sheet in horrible shape and pic of another black vert another member has that still had it's Argent pieces.
To find the reinforcement go to Ebay and search for Battery Box Reinforcement 1973 Mustang. He might not have his add up if out of stock.
Keep posting your progress.
Cheers