The way I understand some of it,vac mod input[they make a aftermarket adjustible one],plus govener WT.s and springs [B&M makes a kit for this]and the kickdown cable adjustmint all have some efect on shift point.
I have not got the $ right now too play by buying these things,so I'm having the same prob myself.
The other prob is I pulled the kickdown so it is at full pull when carb is at full gas pedal,this maybe too much,cus it don't kickdown from high when I step on it hard. My son's T-bucket puts little too no load on 350sbc motor,but I'd like it too downshift.? don't know