Looks great without the decklid spoiler, nice clean lines. Mine came with D80 originally, so that'll stay, but now that I am older I much prefer the understated look of a Camaro without F&R spoilers.
Now back to the spare: I recall when researching the length of the correct J-hook for my car, that there were different factory configurations for coupe, convertible, full-size and space-saver. Unfortunately I can't help with any hold-down bracket location dimensions.
However, a couple of observations on your picture (I realise you probably know all this):
1) I believe the spare wheel was intended to be placed upside-down in the trunk, i.e. with the valve stem pointing down - maybe this helps fit the inflator can?
2) Jack mast was to be installed on a diagonal angle, touching the passenger wheelhouse and the other end fitting in the angle bracket near the forward trunk center.
3) Tire should be touching the seatback and the passenger wheelhouse.