A point needs to be made here. A timing chain won't make the compression readings vary like that.

You're either looking at rings or a valve that leaks on the low cylinder. If it was a chevy smallblock from the '60s or '70s it might be a bad camshaft lobe.

Put a spark plug in just that cylinder and spin the motor. Listen for the leak. An intake leak will be easy to spot, an exhaust leak a little more difficult.
If there isn't an obvious leaking valve, squirt a little SAE 30 motor oil in the affected hole and redo compression test on that cylinder. If compression comes up markedly, it's rings.

If you find a bad exhaust valve the head should come off. Burnt exhaust valves can torch a hole in the valve seat area.
A bad intake valve you can live with better.
A bad ring seal can also be lived with.
There are downsides to any of these "living with it" scenarios, though.

Now another point: Your 8.2:1 400 is really more like 7.2:1 if you were to measure everything and calculate. I'd suggest starting to plan an engine rebuild. Tech advice from this board is free and mostly very good.

R.