I didn't say connect to the chassis. I said connect to the terminal on the engine crankcase where the large negative lead from the battery connects. Electrically speaking this would be equivalent to connecting directly to the negative post of the battery. The engine (or, at least the crankcase) is a valid part of the electrical system. The frame is not. You can complete a bad electrical circuit through the frame because the engine is bolted to the frame at several places. But these are physical connections and not electrical connections.
The starter motor, the neutral switch, and the oil level switch all use the engine crankcase as part of their electrical circuit.
The R/R doesn't shunt any current to the chassis. The shunting is done internally in the R/R. There are only 5 electrical connections to the R/R. The three A/C stator phase leads and the two DC output leads. All shunting means is that the AC stator currents are switched away from the rectifier diodes in the R/R and shorted back to the AC stator windings -- the rectifier diodes are bypassed. But none of this has anything to do with the R/R's physical mounting to the frame. There is no electrical connection between the R/R and the frame.