Have you tried a vacuum test? Rough idle can be due to a vacuum leak (either intake or a hose/valve in the PCV/VCV system). M70s had issues with the water temperature sensors. They had 3 (1 for each bank of cylinders + 1 for the gauge). Bad sensors resulted in the engine running rich even at normal operating temperature and made the idle rough (smelly too).
Also check the fuel pressure regulators (if your fuel pump and filter are ok).
Good luck and I hope you get it fixed!