The way I read the rules are that a rear seat is required, even with a roll bar in Pure Stock. It states rear seat may be modified, not removed, if roll bar is installed. If that's the case, then the only Mustangs that would be legal without a rear seat would be the ones that came from the factory that way, or clones of them, ie Cobra R's and newer Cobra Jets. Send an email to Mike Baker with IHRA to confirm:
mbaker@ihra.com
Please post back what he says, I'm curious to know for certain,