Ya I remember the linkage up by the carb. It used a bolt that threaded in to the rod on top of the rod was a flat piece of metal with a slid slot to adjust the kick down.
If it was not adjusted right it would not kick down at all.
I could swear there was a vacuum line. though my trany was worked over pretty well.
I had the 440 magnum with the 727 and a Dana 60 with 410 gear and posi.
I installed the Dana 60 after I twisted the yoke out of the front of the other one when it hooked up on dry pavement with them N50 tires. When that thing realy hooked it either went or you would see parts flying. HE! HE! HE!