I was guilty of one or two of those faux pas.
Especially #7 in the 71 Mach 1 I owned from 1975 to 1977. I did retain the original radio, which is stored somewhere in my basement. I you're familiar with the 71-71 Mustang fastback design, you know how there's a plastic molded armrest for rear seat passengers. I unceremoniously chopped nice holes into them to install 5 inch round speakers just to listen to Black Sabbath's "Sabotage" on 8 track.
(Still have that 8 track too)
Guy who now owns the car wasn't to pleased to hear about my custom work.
Apparently those molded armrests are/were made of unobtainium. Oops.
That car was factory built with the solid lifter 429 SCJ engine, and more than a few guys were sure the cam and lifters were bad. It had a 780 Holley carb from the factory, so didn't need to change that. I did install a set of Hooker Headers, along with my "custom" exhaust system consisting of 2 sections of straight pipe clamped to the headers and turbo mufflers. Then I hung the mufflers with some coat hanger wire.
That held up for almost two years, until I sold the car.
Never bothered with any seat covers, as I didn't care for them. No air shocks either, just had the factory leaf springs recurved with one extra leaf. That cured the wheel hop issues it originally had.