My tranny is having a similar problem to what has been posted here by others. When the speed of the car reaches 40-50 mph the rpms shoot up to 5000, and the tranny will stay in low gear after that.
I put in some trans-x and the problem seems to have gone away as I only drove it for a few miles.

How long should I run the trans-x in there and should I put more in when I do a filter and tranny fluid change?

Also how many quarts does an 84 300DT take when you drain the torque converter as well. The Haynes manual I have calls for 6.7qts, but according to a friend it should take 3.2 gallons (about 12 qts)?