Who cares if "they" didn't use springers? And, actually, they used whatever they wanted. I've seen vintage photos of klunkers with springers, since they probably tried everything they had laying around.
Most ended up with a bladed fork and truss rods for strength, at least until bmx style cruiser forks became available, probably because of the weakness, mushy feel, and the way springers tend to wash out in hard corners.
A modern klunker type bike can be anything you want.
If you're gonna hit serious trails, I wouldn't go with the SA. Those three speeds have a tendency to shift on their own, or find "neutral," usually when you're putting the hammer down.
But if you're just building a custom cruiser, do what you want.