I wouldn't worry too much about the marks. They always are a little retarded at the cams. If there is a significant difference between heads, it might be worth testing it completely. It's been years since I have done it, but the concept is to dial indicate the valve openning of cyl 1 and 6. In a hydraulic lifter car I think one is supposed to remove the hydraulic and replace it with an adjuster from the older mechanical lifter engines. Then one adjusts all the freeplay out and turns the motor till the valve is depressed 2mm. At that exact point the degree measurement is observed on the crank and compared to the appropriate value for that valve from the TDM (technical data manual).

If it is off 5deg or more then offset keys are available (or atleast they were) to correct. They were available in 2,3,4, and 5 deg. This is cam degree which is doubled at the crank, so it really is 4, 6, 8, and 10 degs. Since a single tooth is 18 degrees one can get about anything within 2 degs by using the offset to advance or retard. In other words I used to see a lot of chains stretched such that the right cam was 14 deg retarded (at the crank). By jumping the chain a tooth it became 4 degrees advanced, then using an offset key to retard it 4 deg it was perfect.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician