LA small block timing is suddenly way off

Hi, I have a 360 small block LA motor in my '67 Barracuda. MSD 6AL electronic ignition, MSD distributor with mechanical advance only, no vacuum advance, electronic points. Early this summer I set the timing by ear by rotating the distributor advanced as far as possible and it was running fine for 2 months. I had trouble seeing the timing mark with my timing light as it came around, so I did not bother noting what the actual idle timing advance was. Engine ran fine until recently started missing terribly (maybe after I revved it to demonstrate new mufflers?). Now I took a black Sharpie and blacked the timing mark on harmonic damper so I can see it with my timing light. From earlier static testing I know this timing mark aligns with TDC on cyl 1 compression stroke. Idle t.Timing now appears to be 30 degrees retarded and misses terribly!! I cannot rotate the distributor any farther advanced because the distributor cap mounting ear contacts the riser of the intake manifold. I think something may have slipped a cog?.. Sorry I don't know if the LA camshaft is driven by a chain or a gear. Is it possible my camshaft (chain) has slipped a cog? Or is there a way to remove some screws to advance the distributor rotor by about 30 degrees so I have some room to then adjust timing properly by rotating the distributor? I am stuck.