I think you will find that springs used 6 Cylinder Mustang and U.S 6 Cylinder Falcons will fit.
The XP uses a spring with 11-1/2" Coils - Part Number C3DA-2035-A
The 65-66 6 Cylinder Mustangs and Falcons also use a 11-1/2 Coil - Part Number C3AZ-2035-A
You'll find these in rebuild kits through one of the numerous mustang or falcon stores online.
Or Google C3AZ-2035-A
https://www.macsautoparts.com/ford_m...and-shoes.html