What you said the spring shop told you is correct. This is how you measure a spring.

If you take a tape measure and go directly from A to B. It should still be either 52 or 56 inches. I measured mine as the picture indicated and got 56, then I went from A straight to B and still got 56.
You said your truck is a 74 K20, my truck is a 73 C20, but my friends truck is a 75 K20, his rear springs are 52 inches, so yours should be 52 also. I believe being a 4x4 makes it 52 inches and a 4x2 makes it 56 inches.