Katman,
These cars actually have a factory installed bar that runs under the dash and is attached to each side... basically a cross-bar like you mention. There isn't a lot of room under there with the dash in place to attach the bar to the uprights of the cage, though I do think it could be done...

Car looks good bbp! The cage looks remarkably like MINE! I'd be interesting in talking with you offline about your car and some of the things you've done... I'm racing mine in RS this season, with hopes that it will eventually get classified in Production. Perhaps we could swap information and help each other...

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Have FUN!