Non-HD transmissions come with several different differential ratios, none heavy duty size, and since most of them use 35/35 or 1:1 gearsets already, you cannot purchase 3.29 gearsets to alter the final drive since they're what you already have.
To change the gear ratio, you'll need to swap the diff, and
since they're easy to swap you may find you want to look at a swap to improve performance or reversely fuel mileage.
97-03 Grand Prix natural 3800s also use a 3.29 differential and 35/35 gears
04-08 Grand Prix natural 3800s use a 3.05 differential and 35/35 gears
05-09 Allure/LaCrosse 3800s use a 2.86 or 3.05 differential depending on year and 35/35 gears
00-05 Impala 3800s use a 3.05 differential except for some sport packages that use 3.29 and 35/35 gears
00-05 Monte Carlo 3800s use a 3.29 differential and 35/35 gears
00-05 Monte or Impala 3400s use a 2.86 differential and 35/35 gears
06-11 Impala or Monte Carlo 3500s use a 2.86 differential and 35/35 gears
06-11 Impala or Monte Carlo 3900s use a 3.29 differential and 35/35 gears
05+ Buick Allure/LaCrosse CXS with the Cadillac 3.6 Alloytec use a 3.29 differential and 33/37 gears for a final drive 3.69 ratio
As you can see from above, vehicles that have a 2.86 differential such as the 06+ Impala 3500s could benefit in acceleration from a 3.29 differential swap, for example, from an 00-05 Monte 3800, 97-03 Grand Prix or 06+ 3900 Impala.
Overall I find the 3.29 gearing the best feeling and best performing, and its worth the swap in either way you need to perform it.