I would do something different - check to see if it will shift properly with vac. You need zero vac for up shifts and vac for downshifts.

You can test this by unplugging the line from the vav to the trans modulator. Hook that line into a Mityvac and run it inside the cabin. Then drive around without pumping up any vac to the trans modulator. The upshift should be smooth. Before coming to a stop, pump up the Mityvac and your downshifts should be smooth.

If it does not work like this, you probably have a vac problem. Of course there could be other things but this might identify one.