Stumpy, usually when shaving a part of a car body you are removing something. I'm not sure what you mean by shaving the cowl. Do you mean that you are going to fill it in so it is solid? If that is case, then you are "filling" or "molding" in the cowl vent.

And Henry is right, the cowl vent is the intake for ventilation for your passenger compartment, not just a rain gutter. Do you have an alternate plan for air flow? Just something to consider before you do it. I watch them fill the cowl vent on Lance Armstrongs GTO in Overhaulin the other day. They just welded a panel in where the vent used to be and smoothed it out. They also installed Vintage Air and his is a convertible so that was their plan for air flow I guess. Rain gutters or drip rails usuaually refers to the edges around the tops of the doors or windows to keep water from dripping into the car when the door opens.

Hope that helps, didn't offer much on technique, but maybe some help on terms.

Pat