1968 340 timing
This is a 360* graduated strip of plastic you install onto your balancer, that helps you ;
set your valves,
find TDC of any cylinder
and set your ignition timing.
You have to align the starting point to true TDC, and make sure it goes on in the correct orientation.
You can get one at any performance shop.
With this mounted correctly and a good-quality NON-dial-back timing lite, you should be able to find your mark, or at least get real close to it...... if you are on the right wire.
By the way; 69 and older 340s have a different timing cover and matching damper, than the 70 and ups. The older ones were timed from the passenger side and the newer ones from the driver side. Or was it the other way. In any case to use the timing tabs, you have to have the matching damper, else you cannot find the damper-mark.
This has already been mentioned; go back to post #12.
But if you correctly install the timing tape, then it no longer matters.