71 GTX, 727 3rd gear won't engage unless I lift off the gas?

That can be a problem with them cables when used for the kickdown. It would not shift into 3rd on normal throttle because it had to much throttle pressure. And now after you adjust it the throttle pressure is lower but to low that it wont kickdown now. It needs the right geometry to the kickdown lever and your problem is what I hear some have with the cable. I would try backing it off about 1/8" and see how it does. Because you dont want to little throttle pressure in the trans as it can smoke the clutches since the throttle pressure also goes to the pressure regulator and it can cause to low of line pressure if adjusted to loose. The one thing to remember is its best to have to much then to little throttle pressure so you dont smoke the trans. To much of course is not right but its better to drive with to much then to little throttle pressure so you dont hurt the trans. If you change it back about 1/8" half of what you moved it before and it still wont work right then sometimes you have to drill a new hole in the lever for the throttle pressure (kickdown) to get it to have enough throttle pressure so it still kicks down and will shift into 3rd on light throttle. Myself I like to put a pressure gauge on the trans also to make sure the throttle pressure and line pressure are right when using aftermarket kickdown linkage. That way once its set right you can make sure the pressure's are correct as I have seen more then one guy smoke his tranny clutches by not having the throttle pressure adjusted right or hooked up. Also I had one friend who mixed and matched rods for his kickdown and when I set it to work right for good upshift with no slippage and good kickdown but then the trans kickdown linkage bottomed out before he had full throttle so I had to change it all around to get it set for the right pressures and have full throttle. Ron