Squirrely while braking

Hi, My 66 Fury has power drum brakes. It brakes fine except when I'm going 50 or more and have to brake hard. The front quickly pulls to the left and right evenly and comes to a straight stop and is startling if you don't expect it.
I've been under the car with my son and had him hold the steering wheel while I grabbed one front tire and forced it back and forth looking for looseness in the linkages but find none. I figured it might be the front brakes not being adjusted evenly so I tried...

Squirrely while braking