Hello 71 Project Junk,
Some of the Ford catalog listings are about as clear as Mississippi mud! The illustrations have either a bolt number that was never released for service or the incorrect number listed, which was never corrected through the many revisions over the years. The text section never listed the correct number, and a lot of times (After several revisions), the Mustang listing disappeared.
The correct bolt is (Ford# 383519-S100--replaced by 384871-S2) 5/16-18 X 1". It is a hexagon head bolt with an S2 finish, phosphate, and oil. The closest Amk product is F-940 which contains B-11354 bolts. Amk requires a $30.00 minimum order with a $10.00 shipping charge. A negative if looking for a small number of hardware pieces.
The long spring, D1ZZ-6561828-A, was also used on '71-77 Maverick (Bucket seat models), '74-78 Mustang II with the 2-way manual seat, and sister Mercury vehicles. Unfortunately, many of these vehicles were "Thrown away" at the end of their life cycle with of treasure trove of these types of small parts no one gave a second thought to then.
To my knowledge, no one is reproducing the spring, which is probably not an often sought-after part. Mike @ Motor City Mustang would probably be your best bet. He has a stable of parts cars, with many parts looking for a new home. If for some reason, Mike doesn't have these, you can try Don @ Ohio Mustang Supply which also has parts cars. Now you know more about these bolts and springs than you thought you'd ever want to know!