I've had similar cars (Mustang) with stiff springs + soft rollbars, and another with soft springs + stiff rollbars. I preferred the soft springs and stiff rollbars. It felt better to me going around corners, plus the ride on smooth pavement was much nicer. It had a monster front swaybar that reduced roll to imperceptible. It would corner faster than I dared. The stiff-springed car was a police pursuit model and felt more conventional in the handling department, and probably held the road just as well, but just felt less confidence-inspiring. On the other hand the stiff rollbars were *bone-shattering* offroad (or on uneven pavement).

But as dattodude says, it is complex science. I'm not sure the same would work on a Datsun 1200 because the rear springs are already fairly stiff, partially due to the light weight of the car.