1974 Valiant Scamp, timing retarding when revved

As said, if the index-mark on the balancer, marches up (CCW from the front of the engine), and disappears under the waterpump, that would be normally advancing and is correct.
If the cooling fan is direct drive, or if the clutch is tight, I would repeat the test with the Cooling fan disabled, and with about 12* of idle timing.
It sounds like she needs more idle timing,more pumpshot,Less idle fuel, and I believe your T-port sync is off/ and or the metering rods are tardy.
But I have to agree, that if the timing mark is in fact retarding (marching CW, down, towards the power-steering pump), That the engine would sound like it does. And I know of no way that could happen; so the first thing I would do is swap out the ECU. I have never seen a wrong-way marching timing mark.So ........I'm arm-chairing it,........ again.
The vibratoryness kindof indicates to me that, if the fan/accessory drive is not the cause of it,I suppose the engine balance might be. That's a whole lotta shakin' goin' on.