I don't add a shim, by doing a total of 3 springs it does essentially the same thing. You can try doing everything but I've never found it to be needed since 4th was never really made to handle any kind of big power. Just quickens it up and gives it a better feel shifting into 4th......Edited my earlier post, it's 2 original springs with the 1 from the shift kit.
I run the shift kit a little differently. I make the 1-2 accumulators stack up the same in terms of springs so the red one currently in the 4th housing (in your pic) gets moved over to one of the 1-2 accumulators and then it end up being 3 yellow springs into that 4th housing.
