All those braces, crossmembers and braces are what “limited” the side rail from twisting like a wet noodle.
But even with all in place it was designed to flex - A LOT !
I assume you will bolt those brackets, such as spring hangers and such, back together - might be wise to use bolts a little longer ( something like the T-case crossmember to frame setup ) with a spacer so the bolt can stretch when needed instead of snapping.
Put it all together with the rails supported by stands and take some measurements BEFORE you start tightening anything down first.