if your tranny is fine you don't need to get another one, as for gears, you can go with a lot of combos, but it depends on what you like,
first, do you know what gears you have, are they 2.56:1 or they are 2.93:1, to find this out you will need to check you spid code in your trunk lid.
second, if you go with higher gears you will want a posi, you can go with 3.08 as a caprice or impala, or you can with 3.42, 3.73, 4.10 or higher. then depend on the gears you get you will have to get a torque converte,
third, as for the tranny, you can rebuild it with better parts adn reprogram your pcm will be a most after, you do the tranny and gears.
be prepare to spend almost $3000 on gears, posi, converter, drive shaft and installation, you can probably save some money on the drive shaft if you get the oem rebalance, but that is up to you.
for a new performance tranny is like another $3000 to $4000 and that is a very reliable source, so you will have to do research on that, you will find very different opinions on who or what company builds the best and DURABLE tranny for our cars.
good luck and tell us more about you caddy