A plate behind the rear seat is effective and simple. It's a common trick among race car builders. I also once read a list of torsional stiffness of BMW's 3 series. The one with a fold down rear seat (and consequently no reinforcements there) was considerably weaker (like 25% or more) than the standard sedan with fixed rear seat, which obviously had some reinforcements there.
There's a thread on stangnet I believe of a guy who took measurements when installing all kinds of reinforcements. He supported the body on three jackstands, bolted a long beam to the front, applied a load on one end and measured the twist.
The rear seat plate improved stiffness by 10%. Not spectacular maybe, but good enough considering the simplicity of the modification.
An export brace improved stiffness by a considerable 25%! A real export brace that is, not one of those "moves freely in almost all directions" heim jointed things....
Subframe connectors didn't show any improvements in terms of torsional stiffness. That is not to say they're worthless though, they do make the body more sturdy.