The tightness you are noticing in the center of travel is normal. Without it, the car will wander when driving straight down the road.
There are two adjustments on a steering box. First, the preload on the bearing on the input shaft is adjusted by the large nut that the steering shaft goes through at the top of the box. This preload creates about 5 in/lb of bearing drag. This insures that the bearings are tight enough to prevent play but loose enough that they do not bind or wear quickly.
The second adjustment is measuring the drag created by the action of the teeth on the sector shaft against the teeth of the rack block. This drag is measured as you turn the box from lock to lock and measure the center. The drag should remain about 5in/lb and then go up to a total of about 10 in/lb at the center, then go back down as you pass to the other side. A box that has no "high" spot at center is worn out and will never work correctly.
With the box out of the car you can just feel the difference when you turn the shaft by hand or with a wrench. It is very slight. That is why I cringe when I hear about people cranking on the adjustment screw at the top of the box. You just can't feel that small difference of adjustment in the car. By the time you can feel the difference, the box has been over adjusted and will either fail or wear quickly.
Be glad your box feels tighter at the center of travel - this means the teeth inside haven't worn out.