i owned a 38 coupe, and my one buddy owned a 37 sedan, plus, i parted out several 37/38 sedans. the firewall is indeed, stock on your car.
when the time comes for the mockup of the /6, consider replacing the pusher fan with a factory [OEM] style puller. since the 6 lays over, you may find by ditching the fixed fan and spacer, the firewall mods may be minimal, and the puller fan can be mounted high, missing the engine. as i don't have a /6 here to measure, i can't tell you the length.
although the original engine was a 6 as well, it may, or may not, be close. i'm saying this because the inline 6 was a heavy type of casting, while the /6 was a lighter, more "modern" style of casting, and may be very close to the original in size.
it was designed to fit in the new A body, and those had not only a lower hood line, but a somewhat cramped engine bay as well.
in the very early 70's, my one buddy had a T-bucket with a slant, and that thing would flat out scoot ! i realize that is an apples to kumquat comparison, but the cool factor was way high.