While looking through a parts catalog, I came across something called a "convertible floor pan support" that is supposed to help tie everything together. It bolts to the bottom of the floor pan. It makes sense to me.
I am about to replace the floor pans on my '68 RS coupe. Would adding this pan support be of any help to a non-convertible?

Thanks :)

Your coupe doesnt have the underbody braces that a convertible has that the x brace you described would bolt to. Get some subframe connectors if you want to 'tie everything together.'

Aha! That makes even more sense. (Can you tell I am still kinda new at this stuff?)

Thank you.