Your assessment of the original Shadows is pretty much spot-on: it IS rougher, with super-short scenarios compared to modern ones (fewer than 10 pages as opposed to 20 or more for new ones). It's also a hideously difficult mother too, with scenes in virtually every scenario that could lead to TPKs, monsters that can't be harmed by weapons, and incredibly powerful sorcerers, to say nothing of the effing murder-chapter. Still, there are lots of good scenes and characters and so forth, so it's worth revisiting just to see how the very first CoC supplement looked way way way back when.
The original SOYS, BTW, is only about 80 pages, not 180. Like I said, they wrote 'em SHORT back then.
And there is indeed an updated/revised/expanded version of Shadows in the works, so the Classic reprint won't be an exact duplication by any means .