I hauled ten, 20 pound sacks of lime in my 1960 TR3 a few years back. No problem, although the suspensions was down a bit more than usual.
The best sleeper hauler I've owned was my 1988 Saab 9000. Fold down the rear seats, remove the rear deck that doubled awesomely as a baby changing table and throw in one twin mattress and one twin box spring in, then close the hatch completely with no muss and no fuss.
Move again. . .throw in two twin matresses, two twin box springs, but could not quite get the hatch closed. Oh well.
Whole sheets of plywood or gyp board would slide into the hatch. In an area above the rear wheel wells, the swedes found a way to make the hatch over four feet wide, so we americans could carry 4x8 sheets inside the car. No, the hatch would not close all the way, but this is a 30 mpg, in 1988, 5-door sedan, not a wagon.
Jack