Timing lights do not work on old engines. The timing marks you are trying to read are set on the harmonic balancer, which is mounted in rubber. Over time, the outer ring of the balancer moves. If you use a timing light, you will not be getting an accurate reading.

The best way to set timing is essentially what several people have recommended. While the engine is running, turn the distributor back and forth until you get a real smooth idle. Then test drive it. If it pings on acceleration or kicks back when starting, retard the timing a bit and try again. If you have a vacuum advance on the distributor, you can disconnect it while setting the timing, or not. I've done it both ways and not noticed a big difference.

Setting the timing like this will clear up alot of problems that people mistakenly attribute to problems with their carburetors (ie flat spots, hesitation, etc.).

I just bought a 72 F250 with a 390. It was VERY VERY sluggish. Just for kicks, I put the timing light on it. According to the timing light, the timing was spot on. However, I set the timing as I discussed above, and now the truck runs like I expected it to. Night and day difference.

 

Wow, good info! Makes you wonder how many carb rebuild kits have been sold to improve the sluggishness.