What causes the car to understeer and oversteer

Did you have the column out if so is coupler put back together correctly?

Even if you didn't have it apart, if it is possible that someone else did I would check it. I had excess play in my '67 Barracuda steering, even after I replaced all the suspension parts. Turns out the previous owner must have pulled the coupler apart and put it back the wrong way at some point. There are two little blocks in there that slide over pins, and there is a spring that pushes them outwards to contact the coupler body. I am attaching a picture I found online. Make sure the blocks are positioned as shown in the picture.

They can be installed two ways, but only one works. It is easy to insert them with the rectangular blocks aligned with the shaft, but that is wrong. They need to be perpendicular to the column shaft so they just slip into the coupler with very little movement, then pack the coupler body with grease and put the seal plate down in place.

Coupler.jpg