The Hennessey HPE550 was designed to be a 'carbon copy' of the plans GM has for the Z-28, so other than the badges and whatever bodywork GM finally ends up going with, you can have all the performance of a Z28 today for just your SS + $18,950
You can even see how it stacks up today against the GT500 and SRT8 -
And just for fun, let's see it's big brother, the HPE650, go up against the GT500 SuperSnake and SpeedFactory SRT8