Welcome,
You are getting great advice from this group.
I recommend that you get a parts car. Ideally you will find one the runs and drives with an engine that you can use in your fastback.
You will have to put some effort in to the hunt but complete and running 71 - 73 coupes can be found for around 3K or less. In your case rust or a bad title is OK and will help drive down the selling price.
This gets you every clip, harness, mount, and all the odds and ends that can stall your project and drive you nuts. Even if you can't use a wore out part at least you know exactly what you need to get.
You may even get lucky and find something highly optioned. Tach / Guages / Power windows / Tilt wheel / are always fun.
I am one of the older members here and have been doing 71 / 73's since the early 80's. No matter what the project is I always get a complete parts car.
When you are done you can sell parts, sell scrap, and pretty much get your money back.
Best parts car finds so far - a pair of 71 Mustang Convertibles for 700.00 each and one turned out to be an M code Ram Air with the Ram Air stuff in the trunk.
Best of luck!
- Paul
