I installed one in my Camaro. Like Graeme says, with a little tweaking, I got my back seat back in. I had to roll down rear 1/4 windows and do some bending and twisting (the seat, not me....well, me a little) but I did get the seat back and bottom locked in their original places/positions. My bar was a Camaro unit, with Camaro mounting plates. I'm told the bars are all quite similar with differences in the mounting provisions. I'd think that if you have all pieces correct for your Chevelle, the rear seat should fit after installing the rollbar.
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