Have you tried a different knock sensor? 70's is super high, but it might be other engine noise. Have you checked the endplay on the motor? Does it actually read zero with the engine not running but ignition on? Something is wrong somewhere, I can't recall ever seeing over 20 even at high boost with a dog box.

If you are seeing it free reving at idle, maybe put a stethoscope near the knock sensor and listen for whats causing the noise.