In addition to Joe's good comment, you might be getting some things confused. Usually the springs that are sold by weight rating are for coilover, spring/shock combinations. "Conventional" springs are sold by vehicle application based on original equipment, just as Joe has enumerated. Since there have been some conversions that utilized coilover's on Mustang II suspensions, just want to make that clarification.

The Mustang II ('74-'78) suspension was the same as Pinto/Bobcat, and ran from '71 to '80. The '80 T-bird was built on the Fox platform (Fairmont, Mustang ('79 & up), T-bird, etc.) and used a modified McPherson strut front suspension. COMPLETELY different than the MII, and the springs are in no way interchangeable.