Since the dash is out on a stand, you may be in the best place to judge if you can get the brackets correctly mounted without detaching the pad from the frame.
The VIN tag, is memory serves, is riveted THRU the pad and into the frame. Getting the left speaker bracket in there, without impacting the VIN tag may take some finesse.
I'm guessing it will take under an hour to detach the pad, mount your brackets, and refasten the pad. I cannot imagine what a nightmare it could be trying to do this with the pad assembly installed in the car.
Pulling/installing a dash pad is one of those things I dread, unless the car has been stripped of its interior for bodywork and paint.
It's a real nightmare trying to R&R a dash pad without pulling the center console and/or scratching something, cutting my fingers and bleeding on stuff, or breaking my back doing this alone.