What do you mean by, "box in the back"? I'm not familiar with that. Do you have an example of what you mean?

I do plan to drive it on the street. I'm not familiar with a gusset, but I'm assuming it means to take a piece of metal that will fit between the back of the ladder bar brackets and weld it in between them so that it will close that area and also prevent it from flexing as you mentioned. Is that right? I've seen people weld a thin plate on the bottom of their lower control arms that I'm assuming serves the same purpose. Would there be any benefit to gusseting both sides while I'm at it?

You can see the gusset on the left of the shock(sorry
its all painted black) but its a wedge welded to the tube
down to just above the lower bar mount to kill the
side flex.... the lower hole where the lateral link
mount is sorta out in space and this stiffens it up..
boxing the rear of the bar mounts helps stiffen them
up... I dont have a pic of that completed.. but a
3/4" wide plate welded to the back of the ladder bar
mounts
EDIT
In this pic the back of the brackets wasnt boxed in
yet... I forgot till later

Here are a few pics of the template for the gusset I intend to create for the ladder bar bracket. Please let me know if you think it will be strong enough. It will be 1/4" plate that will be 1/2" wide against the ladder bar bracket and 1-1/2" wide against the axle housing. I will also be anchoring it to the shock bracket as I notched it for attachment of the soon to be created bracket. These first two pics are of the gusset template without the shock installed.

Second pic without the shock.

First pic with shock installed.

Second pic with shock installed.

Do you think I should weld to the carrier part of the axle housing? The reason why I ask is because there is only 1-1/8" of axle housing between the ladder bar bracket and the carrier part of the housing.

I also have another option of making a square bracket with an oval cut in it for the shock to go inside of.

Also when we mount the shocks we dont normally have
them 90* to the rotation... basically turn that 90*
and bolt it in from the side ... I normally use these
brackets for the lower shock mount

I edited my other post.. take a look back

Also if you want to weld to the cast housing thats
fine but you have to use nickle rod for the cast side..
which ever way works easiest for you(most likely the
housing side)

Thank you for the ideas. I think I'll try to design a bracket that ties to the shock bracket and then gusset the shock bracket.