I'm assuming the car ran okay (besides the leaking gasket) prior to your project. So, it's probably in time now. Not sure if it's a fact but I've heard the M103s and 104s can run being a tooth off.

Now, I may be quite wrong, as I've never been into an M104 but I think the mark (TDC) on the balancer that you have in the center of the opening for the sensor, should actually be aligned with the bent tab just in front of it. The marks are a visual guide for the tech, which would be hidden from view if the sensor was in place. The sensor itself doesn't rely on the marks per se'.

Try lining it up with that tang and see if the drill bit or dowel you use in the sprocket holes line up right with the casting. BUT, like I said, I'm not sure, hopefully someone who really knows will verify this or tell you the right way.