71 Dart not starting after new steering box

Since "you messed with it" the two most probable causes are linkage out of adjustment and you knocked the neutral safety switch harness loose, either at the transmission connector, or the connectors at the star relay

Look at the start relay. Identify the two "push on" connectors.......one of them will go into the bulkhead connector, the other will go down over the rear of the engine to the transmission. Disconnect that one and ground that relay terminal with a clip lead

Be CERTAIN the transmission is out of gear, and it will probably crank.

There's an adjustment on the transmission linkage to get the transmission to "follow" the shift linkage in the column.