You are playing with fire to not at a MINIMUM:
1) Replace the guide rails to the timing chain
2) Replace the timing chain tensioner
3) Have the timing chain checked for stretch

If this were my own car, I would definitely replace the timing chain immediately.

It also would be a good idea to replace those new plugs, as mentioned in the previous posts.

I would check the plugs for oil residue as well. If your valve guide seals have not been replaced by now, they will need to be soon. You don't want your engine burning oil and dumping residue all over the valves and carboning up the rings in the bottom end of your engine.

I would also make sure that your trans fluid is cherry red. If it is not, I would have the torque converter emptied and refilled as well as the fluid in the pan.

Good luck. These are durable cars.

We have several clients who have kept up on the maintenance on their late 1980's 420 SEL's who are approaching 300,000 miles. We also have several whose timing chain and rails have gone at mileages far less than yours, who have had to spend BIG $$$ to have the upper ends of their engines rebuilt.

Ed