The above diagram wasn't right when I installed my T/A spoiler from First Place. Check it before you drill. Cant remember which dimensions were wrong, but when you do it, layout that pattern on top of the deck lid - then see how it lines up with the actuall spoiler. Mine needed to be moved 1/4" to work correct.
Conserning the bumpers: You must use brackets from a 70-72 model for both front and rear. On Cuda's maybe the front can be slotted to move it in closer, but that's wont work on a Challenger. Your exsisting front bumper can be reused, just toss out the reinforcing plate. To reuse your exsisting rear, the bolt holes in it will not line up with the 70-72 bracket, even after pitching the reinforcing plate. So you will need to drill new holes or get a new bumper.