C-Pillar Braces
I think these would be a simple and effective mod that would be easy for you guys to make. C-pillar braces bolt in using the seat belt bolts and brace the c-pillars to stop chassis flex. Here is a link to give you an idea these are for 240sx's Megan Racing - 1989-1998 Nissan 240SX Coupe Interior C-Pillar Bar - At KGS Engineering.