Welcome to the new and improved Proflex / K2 Tech Forum!
If I recall correctly, the rear springs are usually about twice as "heavy" as the front ones. I know one of my LBS's has a front SmartShock, and if I recall correctly, it's got a 225 or 250 lb spring on it. That's way too light for a rear shock.Also, depending on what frame you have, the length and eyelet configuration may be different...