Uhhh, you DO know how to adjust the sector screw, right? This is a common thing to do to any steering box whether it has been rebuilt or not.
The engine needs to be idling. The P/S pump needs to be functioning properly too. The 5/8" nut is loosened by turning CCW, the Allen set screw is then rotated 1/8th turn CW, the test the steering freeplay by moving the steering wheel. Tighten 5/8" nut once freeplay is to acceptable levels. Test drive car. There will always be some degree of free movement. To eliminate all of it will result in the steering actually binding up and refusing to return to center after a turn. This is because the Mopar power steering gearbox is 1957 technology! They ran with the design until the last of the Diplomat/Gran Fury/5th Ave cars in 1989. It is not a bad design, but it has its limits. Try as you might, you will never match a rack and pinion for reduction in free play.
The firmer the steering is, the more evident that play will feel. You probably didn't notice it as much with the OVERboosted original steering box because once the tires actually began to turn, you didn't feel much resistance in the steering wheel. The Firm Feel setup makes the steering harder to turn which I'm sure you already know.