There is actually a detent cable and a vacuum modulator on the th350. If you remove the detent cable, you will loose the automatic down shift function.
Now the other problem is with the vacuum modulator. When applying boost it can make the upshifts funky.
B&M used to sell a box that replaced the modulator and had a cable attached to it. Eric Schertz at Dynotech had told me about it before. Not sure it it is still available. You will need a different trans crossmember, as well as a high stall convertor and some other incidentals to do the th350 swap. Nothing is cheap. If you were planning to do a th2004r, then spend the money now as you wont be able to recoup all those things needed to get the th350 going.