I once had a 73 Eldo. The transmission had a chrome hex plug with a central hole. It commonly puked.
I drilled the hole slightly larger and silver soldered a straight brass tube fitting about an inch proud. (Maybe you have mine???)
Installed a hose up to the battery area and open to atmosphere.
Never dropped again.
BTW, based on your brand of lubricant, some create more foam or bubbles than others. The purpose of the hose is to allow the bubbles to rise up, burst, and liquid drain back into the transmission.
In order to prevent water intrusion, it is very important that the hose have a gentle U-turn and point directly downward. Don't spray it with your wash hose.