Mark - with the vac line removed from the modulator you should get perfectly smooth upshifts but crunch downshifts.

You actually can take a Mityvac and plug it into the vac line running to the modulator (block off the line going back to the brake booster) and drive around. I have done this. With no vac you should get great upshifts but your downshifts will be hard. Then while driving pump up the Mityvac and come to a stop. You should get great downshifts.

Well, here's where we are today. I spent the afternoon jacking up the front of my friend's 300D. Found vacuum line and connector solidly fastened to the modulator. Also the driveshaft, center bearing and flex-discs are solid. But it is going to need a tranny mount and front calipers.
Lowered the car and spliced a vacuum 'tee' to the existing small vacuum line. Then connected that to the modulator control on the IP. The violent 'machinery wrecking' shifts are gone, replaced with an acceptable firm thump. Also tightened the alternator belt so now the mad shrieking is also gone.
Plus, I fixed his lawnmower!

Happy Motoring, Mark

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DrDKW