Installing rear torque boxes in 70 E-body. I’ve seen pictures where they have two reinforcing plates sandwiching the forward leaf spring hangar attachment area. Anyone know the thickness of those particular plates?

These plates?

https://arengineering.com/products/reinforcement-plates-ae-body

Yes those plates. Looks easy enough to make. Would like to know what thickness of material was used for plates.

3/16" would be plenty, but 1/4" would be most likely easier to source locally.

Write the check, they will be at your door before the check clears, and Andy will be able to eat this week ...

that is correct. ^^^^^^^^^^^^^^^^^^^
by the time a guy sources the material, cuts the plates to size, measures and lays out the bolt patterns, drills the holes, remove any burrs, clean/prep/paint the plates, you are looking at way more than what andy is asking for his plates !
if the material is on hand and one is a fabricator and enjoys doing everything himself, one could entertain fabricating those plates.
if not, buy andy's !

Ok, but should there be two plates that sandwich the spring bracket attachment structure? (A flat plate without the center hole welded to the outside structure and the plate with the center hole welded to the inside).

So reinforce the base plate but leave existing studs the same?

one can use two plates, but before you start, make sure your studs are going to be long enough to have at least a couple of threads protruding through the nuts when tightened down.

Just did this very thing with 2 plates on each side on my Son in Laws Dumpster to fit 275's in the wheel wheel better.