Calling all 700R4 Experts!

[mister.freeze]

1972 K5 4x4, 350 SBC (300hp), 700R4, NP205. 33x12.50xR15 tires. Stock differential gearing. Torque converter lockup NOT hooked up. OK, so here’s the problem:

I’ve had this setup for years. It was never very good at downshifting, and I suspect bad geometry of the cable linkage on the carburetor. TV cable properly set, but, as I said, the bad geometry doesn’t pull the cable hard/far enough for good downshifting based on throttle position. The issue now is 4th gear (OD).

Normal driving is generally under 50 mph. My daily commute is 12 miles one way on city streets. I rarely get fast enough long enough for 4th gear to engage. This transmission was built about 7 years ago, then stored for 5, and I’ve been driving it daily for almost 2 now. Sild 1-2 shift and much softer 2-3 shifts. Rare shifts up to 4, but it was there any time I was on the highway.

I changed the fluid and filter 4 months ago (my first mistake, right?), and had normal operation for 1500 miles. The fluid was darker than new, but not burnt or smelling bad. Then I noticed frequent 2nd gear starts from lights and stop signs, so I added some Lucas Transmission oil (1 bottle). Shifting improved quite a bit, but was much delayed when cold. This is when I noticed no more upshifting to 4 at any time, even on the highway.
Here’s where it gets weird. Under normal power, a manual shift from 4 to 3 yields no change. However, when coasting, a manual 4-3 shift results in an actual downshift. ??? I understand that without the torque converter lockup connected, a shift into 4 can be almost undetectable. Would connecting it fix this problem with 4?

I also suspect that the TV cable might not be returning as smoothly as possible. Might a new cable be all I need if this is the case?
Thanks in advance for any help.