Dad bought a new Cadillac Eldorado convertible in 1976. GM touted it as the last of the big droptops, a majestic parting shot seemingly designed to transport prom queens, grand marshals, and Boss Hogg types into an increasingly ­dystopian, gas-rationed future. Dressed in triple red with every option, including fuel injection, automatic high-beams, power everything, and a hard tonneau “parade boot,” that 18.7-foot luxo-barge weighed 5231 pounds and was motivated via an 8.2-liter V-8 under a hood so long you could land an Airbus A380 on it. No one can say that Cadillac let the convertible go g­ently into that good night.