66 dart 904 tranny problems

Reverse and 3rd gear going out simultaneously generally points to the front clutch pack. Either a blown seal or worn clutch plates. Either of those means a trans. rebuild. As far as the leak you'll just have to climb under it and find out where it's leaking. No way for me to guess that.

Can't say why taking the throttle pressure linkage (what you called the kickdown) off made it work. That's not a good thing to do as that is the pressure regulator that regulates pressure to the clutch packs. If anything it will make a problem like you described worse and if you run the car with it unhooked it'll fry the trans. pretty darn quick.