kickdown linkage

Caveman is right. Stockers are like this alot of the times.

The thing I'd check first though since you have a non stock type carb is where does the linkage hookup. I found on my 750 Holley I had to hook up the throttle linkage lower on the carb. than the trans. linkage or it wouldn't allow full throttle when the trans. was adjusted right.

If the throttle linkage is ok on yours the following mod can be done to the trans.

If you drop the trans. pan you can adjust the throttle pressure valve (kickdown) stop to hold it in a little more at idle and that will help alot so it don't shift into gear at such a low speed and best part of this mod is it don't affect the high speed at all. You just have to be careful not to go too far or it will cause it to delay too long then you have the opposite problem you have now.