Interesting.
That photo shows a Mallory YH advance mechanism.
There are three adjustments that can be made internally.

Timing for your engine should be close to stock since its a stock engine. It can be tweaked from there but that's a good baseline.
Since yours is slightly hot rodded, looking at non-CAP '67 Plymouth specs, the initial timing should be 12.5* BTC. It should advance 6 to 10* by 1500 rpm.
After that it depends on whether it was the 2 bbl or 4 bbl version, and if we look at '68 specs 4bbl w/ (CAP) manual and automatic are different. What did your '68 start life as?