I am not familiar with your trans but .... Does your trans have a vacuum modulator? Does it have the external pressure adjuster on the side?
Someone posted several months ago that in checking the vacuum line to the trans, they found the rubber connecting piece hard and leaking. They simply replaced that three inch piece and found it made a significant difference.
I did take a careful look at the externals when I was doing the filter change, and it all seemed visually okay.
And a vacuum bug would bite in ways other than just the 2-3 shift, yes? That one flare seems to be the only thing wrong on this unit.
I did finally get a confirm that the K1 spring kit will be installable from below, and I am going to throw one in to my next parts order and give it a try. It'll be a few weeks -- have to fix an overheating issue first.