I would mock things up. You have many other issues to work around such as brake and clutch pedals / linkage, steering shaft from column to rack and pinion. Each piece of the puzzle affects other pieces of the puzzle.
I agree that it needs to be mocked up. I guess I'm curious as to how radical the modification has to be. Having built a Factory Five Cobra, I am scared there will be no room for the drivers feet and 3 pedals. (ask me how uncomfortable that is for a 6'-2" guy) Having measured the width of the LS3 (I believe it is similar to a 350 in size) it comes out to around 23-24" wide. If you recess the firewall that wide, it eats into where the accelerator pedal should go. It would be peace of mind if I could find someone who fit the engine into the stock engine bay without pushing the firewall back. I'm probably over thinking this at this point…
Jerry
