As said, C's didn't have a tach as standard, but the cam had the fitting slot to drive it, so one could be fitted.
I'm not getting what you say though. The standard tach drive is "right angled" and directs the cable forwards and upwards to the clocks. I've never seen one where the cable would foul the pipes. Has someone fitted the tach drive at the wrong angle, ie, pointing the cable straight upwards, maybe?
If you look at another bike (even a pic) it should show you where yours is going wrong.