Your mileage may vary, as they say... I subscribe to the "moderate spring rate, high roll resistance" theory. Anti-sway bars can provide any amount of cornering stiffness with very little penalty to ride quality. Trying to stiffen cornering purely via high spring rates will result in a harsher ride over bumps. if all you do is cruise on smooth asphalt, you may never notice any difference. But if you like to explore back roads, retaining as much suspension compliance as possible will help keep your tires on the road, instead of up in the air where they have zero traction.
That said, there is very little weight difference between a slant six and a small block V8. The reason the factory used higher rate torsion bars on V8s is because of the perceived customer preferences. 6cyl. buyers were assumed to have little interest in going fast. So an upgrade is probably a good idea; .890 might be your minimum starting point. You might also think about the rear springs — a stiff front and soft rear may not be the ideal combo.
