These springs allow your Classic Mustang to take advantage of the latest suspension technology and perform more like a modern sports car. They really do give you the best of both worlds. A fairly stock smooth ride during normal driving, but as the springs compress under load (primarily during braking and cornering) they progressively become firmer very quickly to offer the handling characteristics similar to our 600 pound performance springs, but without sacrificing ride comfort. These progressive rate springs are suitable for long pleasant family road trips as well as a variety of autocross and race courses. These are a little more money, but well worth it if you are looking for the best money can buy. Made exclusively for us in the USA by a premier OEM supplier (for strictly race car applications we still recommend our Performance Springs). Our Progressive Rate Coil Springs feature: Hot wind from SAE5160H, oil quench, temper, water quench, shot peen, preset, 100% load test, ground flat and black powder coating. Note: 1970 351C cars can choose either Small Block or Big Block springs based on car's options. Cars with heavy factory cast-iron intake, PS and AC might be better suited with big block version but can chose Small Block version as well. On all other 351C applications Small Block version is recommended as car may sit higher than preferred if there is not enough weight over the springs.