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I would change out the spring first, change one thing at a time and test drive. Having rebound adjustable shocks is usually a good thing but if you are still unhappy with the ride and assuming all the remaining components of the IRS are in good shape then I guess you could investigate shocks. Do you have a spare tire in the tub. On my 66 vert, I did notice that the added weight of the spare took some bounce out of the ride.