Justin -
It doesn't appear that you can change out to a solid rear in Production.
GT lets you do it, and it's my understanding that this modification was permitted in the 1960's to accommodate Corvettes and XKE's.
If that was indeed the intent of the rule when it was adopted in the '60's, you might have a solid argument for a rule change, seeing as so many more production vehicles have independent rear suspension today, but as the rule reads now, I doubt if Production would permit it.
Chris