There is probably no more important safety improvement for your Amphicar than to add the steering box bracket. It is still a mystery as to why the factory over looked this important improvement. This will stop the movement of the steering box due to inadequate bracing at the steering box mounting. This movement will eventually weaken or cause the steering box mounting to fail. To test this, have a friend turn the wheels back and forth while you observe the steering box. Scary? Yes it is! There are many different ways to accomplish this, the following is my version.
Make a bracket as shown below.
Mount the bracket from the steering box to the steering box mount as shown below.
Use 2 of the original bolts on the steering box and 2 grade 5 (preferably Grade 8), 3/8" bolt, lock washer and nut, for the lower mount.
Optionally, you can weld it in place.