Like VIN7 and I mentioned....the spring rate in the throttle valve affects the pressure rise in the system which affects the shift points. The T.V. system was calibrated with the stock T.V. spring. You replaced the stock spring with a spring with a much higher spring rate. This upsets the T.V. system calibration which affects the shift points. With the higher spring rate...the system thinks the throttle is opened more than it actually is which gives you later shifts. Put the stock T.V. spring back in.
I already mentioned I was having the late shifts problem BEFORE replacing the TV spring, dropping the pan and changing the fluid & filter. I followed the exact same TV cable adjustment procedure before the tranny service and after. The spring is supposed to be 1.90" and the one I put in is exactly that. My shift problem was unchanged after changing out the fluid/filter/TCC solenoid/spring. The Sonnax spring was the same stiffness, the same wire diameter, as the stock spring...only it wasn't compressed about a half an inch by 30+ years of mashing the accelerator pedal. I appreciate the advice and good info you've shared, but since the shifting problem was unchanged by the spring replacement, logically it doesn't seem relevant to my particular issue.
After the TV cable was adjusted about 2 notches ahead, the test drive seemed to have the car shifting and behaving as it should. I'm pretty much done with the car for the season; next spring I'll buy a new TV cable and measure the difference to see if/how much my 30+ year old cable has stretched, and go from there.