This has been done many times over very successfully including my own 69 bird. I would suggest you go onto
www.LateralG.net and do a search on F-body mini tub rear seat modifications. This is an easy fix/solution. There are multiple photos where to alter the factory seat frame, material, exactly how to do so successfully. You will have no issues with the modifications necessary if you desire to use factory rear seat. I did mine with DSE mini tubs with a fold down rear seat and in a convertible.
Tim john---