how far back does the big block in your a-body sit?

just curious, because sometimes i think my engine was installed rearward of the "proper" location.
I want to get some headers, but the engine's fore/aft position may affect that, and i wanted to check on this BEFORE i purchased them.

mine sits with the back of the mopar aluminum valve covers about an inch (if that far) from the firewall....actually...that's the passenger side....the driver's side sits right against the wiring block.