If you had a GT, then changing the sprockets will only get you a worse gear ratio. Also, if you had a GT, you had the best gearing you could get shy of doing 3.69 gears. I'm not sure what was involved in making that work.
Also, with a standard 4t65e, you could swap the diff to change the gear ratio. 3400 powered w-body cars got a 2.86 FDR, 3800 Impalas usually came with a 3.05/6 FDR and the GPs and MC SS got the 3.29 FDR.