Shocked when I weighed my '68 Barracuda convertible.

Considering you went from a /6 to a V8, convertable instead of a hard top, bigger, heavier rear, with a spare tire and full tank of gas, not really surprised. Have you weighed a gallon of gas? Minus that gallons weight x's the amount in the tank. That should add some relief.
Weigh the spare tire as well.