[particlewave]

Just rough measurements...

The firewall is about 2" behind the door frame. I got 26" measuring from the top, from the axles to the the firewall (including the protruding area between the seats).
At the transaxle output shaft (not 12" above) to firewall was roughly 31".

Maybe someone that has their engine bay open can get a more accurate measurement, but these shouldn't be more than 1-2" off. I don't think you'll get 31.5" at 12" above the transaxle output shaft without modifying the firewall (some have wedged an LS3 in there, so should be possible).