The “dead spot” that requires constant and considerable steering corrections to even stay on the road is not normal. I see you had other posts on your resto; did the car do this before you replaced some steering components? If not some possibilities:
-There may be a problem in the box when centered. Even new ones can have problems, especially rebuilts.
-Though rare, the pump may be defective with too much or not enough pressure and the problem is only evident at straight line driving.
-Tires, yes, they are new but a belt can slip or break & cause control problems and severe vibration. As a check, make sure there are no bulges or flat spots anywhere on the tire.
-You did not mention checking or replacing the idler arm, center link, inner & outer tie rods, ball joints, or control arm bushings. These are wear items that can introduce a lot of slop into the steering. Just something to consider.
-Depending on the extent of your steering rebuild, the alignment, (caster/camber & toe-in/out) could be so out of spec as to cause the problem.