A 75 Cadillac Fleetwood 4 dr. Limousine 9 pass. had a shipping weight of 5862lbs. Dont know if thats a record but its pretty stout

One of the heaviest fast cars was a 66 Olds Toronado Deluxe, 4779 lbs. Shipping weight-425 c.i. 375 hp. front wheel drive.
Those lead sledds would suprise lots of people. They would hook up well and and pull like gangbusters.

The 75 plymouth Grand Fury Group, All 11 of them were heavy loads. Anywhere from 4205 to 4930 lbs.shipping weight.

Some more bits of useless information.