Ok, let's step back. What is your current compression ratio? If you have 68 heads and are running 10+ to 1 on an iron head stop now. Buy aluminum heads.
Why are you looking at timing specs from 1968? You cant buy 1968 gasoline anywhere, and it didn't have the alcohol we have in ours today. Those numbers are potentially way off today.
Step 1: Disconnect the vacuum advance. Step 2: Verify your timing mark is STILL at TDC. Yes, in 50 years many of the harmonic balancers rubber rings between the hub and weight dry up, allowing the timing mark to move where it shouldn't be. I recommend buying a new balancer with etched timing marks for about $60, or you can try to determine top dead center with an indicator placed thru the spark plug hole and hand rotation the engine. Once you are SURE you know where TDC is, go to step 3. Step 3: Set the initial timing at 10-12 degrees and then read the total timing off the etched balancer you added or a $6 timing tape from Jegs.
Retard the ignition until it doesn't ping under load, then a degree or two more. Check your total timing. I can only run about 32 degrees on premium gas with my 9.5 to 1 455.
