I have a 70 Cuda with a lazy tach too. Its a rally dash tach and its running off a MSD 6AL with a MSD Ignition 8920 tach adapter. At idel The tach shows the rpm fine, but when you rev up the motor the tach is lazy/delayed. I called MSD and they said one possible fix is too add a capacitor 0.01µF 1 k Volt between the MSD Ignition 8920 tach adapter and the tach.
Any one have a similar issue?