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Acid attacks steel just like electro-plating - the material is deposited disproportionate to the surface area. It removes the most material from edges, corners and the ID of holes.
It removes the least from flat surfaces, like all panels.
This is the very worst thing from a structural standpoint, since the weight loss is minimal until the corners and seams fall apart.

The entire panel must also be neutralized (baking soda, dilute lye) after thorough rinsing with water, or the remaining acid will completely destroy the panel. Submerging absolutely won't do it. Even cars carefully washed sometimes don't completely flush out (direct path from the acid to the exterior). For the final rinse, boiling water from a pressure washer is best.

Sandblasting works OK until your operator forgets to angle the nozzle or turn the regulator down, then you have a washboard instead of a hood. The bondo weighs much more than the weight saved.
There is also no such thing as "effectively masking the area" to allow blasting on assembled pieces. It never, ever works, and you can get sand out of it 5 years later.

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Back in the day a company called Aero Chem in SO CA used Hydrofloric(not hydrochloric) acid to chemically mill steel for a lot of SO CA racers, the air quality enviormentalist got that banned in the L.A. basin years ago They bought a plant up (in El Mirage) in the high desert later, if you knew someone that worked thier you could get them to still do it back in the late 1980 and early 1990s that company did a lot of the aerospace milling back in the day, the racer stuff was secondary only

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Taking weight off the back of the car is the wrong place to do it. Of course that is exactly what I did 35 years ago. On my '68 Coronet I built rear wheel tubs out of sheet steel and in doing so I lost the trunk hinge support structure. So I pinned the deck lid (after I cut out the webbing and spent hours repairing the damage that caused)in the front corners with 1 pin in the center of the back. This meant that when I had to fuel the car, I had to pull the pins, pull off the lid and find someplace to lean it while I filled the fuel cell. I also took the quarter window regulators and glass out cause the new tubs interfered with the regulators and replaced them with plexiglass. I took a few small pieces off the front of the car and moved the batteries to the trunk. So it was a lot of work and no real benefit

The winningest bracket racer in this area is Don "The Worm" Elgin and he adds ballast to his Pontiacs to dial them in. If you need the last possible advantage to reach your ET goals then have at it. If you want to win bracket races, refine your existing combination

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No doubt, that's why I phrased my reply like I did. The intent wasn't to say do it here, but to say look around before going totally radical with acid. My car was "race prepped" by the shop of a known factory racer, and they treated the stamping lines in the sheetmetal like "cut here" lines. The trunk lid was just the most obvious in a photo.