My turbo 350 transmission doesn't shift into 1st until you have sat at a light for about 20 seconds? The fluid level is up, I don't think the linkage position would affect it?
In automatic mode it wouldn't stay in low past 3000 RPM under full throttle, in spite of adjusting the linkage which doesn't seem to make any difference.
Sounds like it could be a governor problem. If theres room you can change it with the transmission in the car. The weights could be sticking or worn on the pivots causing the transmission to not shift properly.