First welcome to the site and hope you find the support and information you need for your restoration project.
As mentioned you have an incorrect fuel line as you suspected and someone is selling it as a one size fits multiple applications. Have never had to buy fuel lines for the car so will leave reviews of other items and providers to them
You will find the search feature here works about as good as any forum search engine. It can turn up allot of information if your take your time. In this case I found one thread showing the fuel lines for 1969s from front to rear. Not sure if it is directly helpful since you know your new one is wrong but may help out others that find this thread later
https://www.concoursmustang.com/forum/index.php?topic=26495.msg163063#msg163063While we're looking at your car and we're focusing on details wanted to mention that you should likely check to make sure your rearwheel wells have sound deadener applied at all or enough as it was applied before body color not after like you have likely have seen all over the internet and done incorrectly