Don't assume the gear box is good. I've put two in my 73 charger, and I need a third.

Unlock the steering column, and grab the steering shaft above the gear box. Rock the shaft back and forth, feel for play, watch for the shaft to move in and out.

Slop back and forth, or movement in and out, and it's bad.

If you do get slop (back and forth) make certain it's not in the linkage.