Your car is more reliant on timing than almost any other MB due to early emissions attempts with carbureted motors.
They run much better anywhere from 5-10 degrees ahead of factory settings (using hitest gas). Factory settings are a tough answer as their were many combinations of distributors in that era. I presume the original timing would be something like 8deg ATDC with the vacuum lines attached AND WITH vacuum in the retard line. THIS IS CRITICAL. If the vacuum is not there or the vac retard unit is inoperative you will loose ten degrees you can not afford to loose. SO, once you have set it to 3 deg ATDC to 2deg BTDC (5-10 degrees ahead), shut the car off and disconnect the vacuum lines and cap them. Now check it again with the engine idling. You should be around ten degrees higher 7-12deg BTDC.
With the lines still off rev the motor to atleast 3000rpm and verify that the timing advances to at least 30 deg BTDC. Stop the motor and connect the lines and verify that the timing is moved back by the vacuum.
How high you take the timing depends on your driving habits. If you drive in town and do it like an old lady, use the max. If you drive on the hyway a lot at high speed; 70 and above for that vehicle don't do the max.
Bosch Master, ASE Master, L1
33 years MB technician