My mates father did something similar. He worked as an electronics engineer and was able to create something that went between the datacassette and C64, I think, ripping the audio in near perfection to another modded tape recorder.
I agree on moving the heads to record certain stuff too, I used to do this, I remember this being very time consuming, but it did work eventually if you were patient enough. Ah! The good old days. Now it's too easy + it can be done in a split second, or near enough.