Once the side armour is off, you can see that the whole structure of the M3 is built from the floor up, with each part reinforcing the next as its assembled. Lots of overlap parts to help strengthen the hull.
here, the sponson plate and the forward armoured mud guard is securing the bulkhead between the troop section and the cab. You can just make out the folded metal of the bulkhead structure between the armour plates.
And the support brackets….just tack welded into each other and then tacked to the upper hull rail, I suppose that once the whole thing was bolted together it could withstand the .30 cal machine guns firing from the rail, but, once the armour was off, I needed to clamp it to keep it from falling apart under its own weight !