Since it seems you have been screwing around with this for a long time now,I'm sure you have tried this,but I have to ask since I didn't see you mention it.
Have you tried to adjust the rear brakes up tighter? If the rear drums are too far out of adjustment,it will have the same symptoms.
Just checking.
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‘63 LeMans- ‘69 400 w/ original transaxle. 2.69 gears.
‘55 Catalina
‘62 Mercury Meteor-all original, bought new by my grandfather
71' GTO -original 400/4-speed/3.23 posi
13.95 @ 102.1 on street tires @ 4055lbs.-now my sons ride