Shocked when I weighed my '68 Barracuda convertible.

My Dart weighted 3420 in full street trim with me in it, that was with a iron headed SM 904 trans, 8 3/4, chrome molly roll bar full exhaust spare tire jack fiberglass hood and front bumper.
Seemed too heavy to me, are Dusters ,Demons and 67-69 Valliant's any lighter than the Darts?

Yes they are. Since they are 3" shorter, the 108" wheelbase cars are always lighter.