Steering All Over the Place at 55mph+

The spacing of the upper control arm mounts for an A body are narrower than a B or E body so no B/E body upper control arm would fit an A body car.
The 1973-76 front disc A body cars used an upper control arm with a larger ball joint that was the same unit used on all B and E body cars.
When doing a front disc swap into a front drum A body, the 73-76 disc upper control arms are needed. The geometry is the same between the drum and disc upper control arms.
Yes, the 1967 models were the last to use the inferior idler arm mount. 1968 was the start of the improved design but usually, a bad idler arm is mostly a problem in terms of precise steering. Unstable steering can often be blamed on bent parts or a lack of caster.