There is no relationship between a 77 300D tranny and a 85. Totally different transmissions. The 77 is only controlled by throttle linkage. You should lengthen the link going to the tranny. Its kind of hard to get to but the adjustment is behind the alternator. The link is around 3 feet long and has an adjustable ball stud at the forward end (behind the alternator. Without driving its hard to say how much but I would start with three complete turns LONGER.
Bosch Master, ASE Master, L1
33 years MB technician