Now again, just throwing something out there I don't understand. Wouldn't more arch have more sideways movement? I would think the straight spring of a 69 F body would have less sideways movement than a large arched spring because that large arch can "swing" side to side at the bottom of the arch like a pendulum. Even if the spring is a mono leaf as I think it is on that 69 Firebird, wouldn't it being flat make all the difference as far as side movement of the rear axle?
I personally am not a fan of poly bushings because I like the cushy ride and lack of noise of the rubber. On my Gran Sport I have poly on one side of the sway bar links for instance so I get a little bit of stiff without making it totally stiff with the poly at both ends.