That part of the chassis member is a thinner plate if I'm not mistaken. The structural one goes to the outside and top and bottom. Not that critical, but as RiverPatrol suggest, the tub will need to come out for a proper fix.
Regarding the chassis number, it does look like a 73. The closest in the registries is a 72 from Australia at G60-30358, so 4000 more vehicles produced will make sense to place yours as a 73.
The ring and pinion condition is difficult to see with that one picture, but probably Rafael's experience can spot to a good approximation the condition just by looking at that alone. For sure he will need more pictures.
If the front axle was fitted with locking hubs, the condition of the ring and pinion will be a lot better in that case, and is probably your best bet. Also, it wears on opposite faces of the teeth compared to the rear, so a basically no wear. Only a little of backlash that won't affect anything.