The vacuum modulator controls the 2-3 shift. Ford had a system of color bands on the modulator to signify heavy or light vacuum signal control. Most builders prefer an adjustable modulator. Pull the vacuum hose from the rear of the modulator and inspect the inside of the nipple for a bladed screw. If there is a screw in there, turn it clockwise and see if that makes a difference. If no screw, replace it with an adjustable one.
Blowing fluid usually indicates over full or over heating. If the fluid is blowing out the vent and is foaming, very hot, you need a bigger cooler. Keep blowing hot, foaming trans fluid out the vent and right out onto your hot headers and you will need a fire extinguisher or good comprehensive insurance.