I think this is real simple; but the cure is not cheap.
I think it could be a combination of too much scrub radius and the lower control arms not set parallel to the ground-plane. Here's my thought;
the scrub radius is causing the tire to pivot and drive away from the centerline of the car. The effective length of the Lower control arm changes, allowing the camber to change, which allows the wheels to toe out, and all this monkey-motion cause the ride-height to change..
I have seen this several times with the powertrain out; when there is no weight to prevent the tiny force generated by the scrub radius to resist this action. Once the engine went in, this action seemed to quit, but that scrub radius problem was still there and could now be felt while driving, as a nervous twitch, and a wander. The only cure was to restore the scrub radius back as close to zero as possible.
If you don't know it, scrub radius is a point in the tires contact patch, created by an imaginary line drawn thru the upper and lower BJs, down to the floor; relative to the centerline of the tire, at the floor.
In the factory suspension engineering, this point is very close to zero, when
the lower control arm is parallel to the ground,and
the camber is close to zero, and
the wheels have a Zero offset, and
the tire is of a specified height.
Changing any one of those changes the scrub-radius.
You have, I think, changed them all.
