Only 6? I think they go up to 15, and no, you can't just add more springs. There are several elements of the transmission that affect how it shifts, and changing one without another could lead to problems. There is a varying number of clutch plates that could have been installed, and there are a few different front band types, along with the kickdown lever ratios, which probably has the biggest affect on the 1st to 2nd shift. From what I've seen, higher ratio kickdown levers usually have more springs installed. The springs release the clutch, but I am not sure if adding more slows down it's application. Unfortunately, I don't have a table with what to use with what, but I do know you can't just change one thing. You have to take the entire transmission into account and know what affects the application and release of one band or clutch, and another.