The old spoiler you have looks like the MAS brand (out-of-business a few years ago). Although it is far from the factory type, it can be bondoed/molded to the trunk, as well as cut at the outer ends and bondoed/molded to the rear fenders (a 3-piece type), like on GMs and others. I'd search for a 70 trunk lid which already has the factory cut-out rectangular holes on the under-skin, then simply spot and drill the bolt holes.. just like an original. I'm pretty sure you can buy a new repro 70 trunk from AMD or others, and fairly reasonable $$.