Longer springs don't need to have longer shafts but it will stiffen the rate if compressed. So longer would be better but the down travel will be a factor cause the arms will only drop so far. I would stick to spec springs and get the rate you desire........
EDIT...: THE SPRING RATE WONT CHANGE UNLESS ITS A PROGRESSIVE SPRING. Only way to change the spring rate is by changing the spring strength.
Longer may fully compress quicker depending on the length.