Didn't see anyone answering this so let me give it a shot.
Assuming you are going to paint it original factory colors with original style stripes, IMO the best way to separate it is a clean line vs. the haphazard crap they did at the factory (anyone with an original
H/O can see the paint separation lines under the decals if they look real close. Not even remotely straight). What I told my painter to do is to paint the whole car's silver part first. Then use a strip of 3/4" painters tape to follow the top of the body break line and around the wheel openings (the stripe starts about 1/2" away from the body break line, so you should be good), then just run another strip on top of that, then remove the lower tape strip. Then mask off the top of the car from there, then paint the black. The 3M rock guard was behind the front wheel well, along the rocker and all the way back stopping at the rear bumper. There's some that argue it wasn't on the front fender behind the front wheel well, but it was originally there on my 84
H/O, and all the G-body 442s I've ever seen. At any rate, the rock guard was not applied in front of the front wheel well on any of the cars. Again, a 3/4" tape line from the bottom of the body break line this time, so as to give the rock guard a straighter line. The factory really sucked at doing this and usually was wavy there as well. Some people opt out of the rock guard, so whatever you want to do works.
Make sure to document how your black paint looks inside the door jambs, take pictures and have your painter duplicate that. Factory was sloppy, but you can have them neaten up the lines.