Unless we've found a new one, we no longer have our long leaf.
Yes, the add-a-leaf kit for the 3.5" springs is longer than the, well, shorter one, it's still not the same as we used to offer.
It might still get you where you need to go though Ol Mule, without killing your ride. But you were right to question this, as length can be pretty important.
The other issue you might run into when ordering a single leaf to fit a particular pack, is that the way the different manufacturers design and build the springs might put the new leaf in contact with a rivet head, or clamp area, when you really don't want it in those areas.
You "can" install them this way, but that's not optimal. The better method would be to find the location where they are closest to the proper progressive length, then cut an end or two off to let them sit flush with the adjacent springs. Smoothing and even tapering the ends is a best practice too.
What makes this all so difficult to achieve (sometimes) is that most of our 9, 10 and 11 leaf packs do not leave much length variation between one leaf and the adjacent leaves above and below it.
I'm sure someone can get you an exact measurement of their leaves though, and you can use that measurement to see if they will fit into your pack in a smoothly progressive transition.
Paul