Rebuild what? Rebuild the car (referred to as a restoration rather than a rebuild); the engine, or the transmission?
If your car is stock it has a 4L65e in it. If the previous owner broke the one you have now drag racing it, and you wish to do the same, then replace it with a 4L80e with a new 32 spline output yoke (you currently have a smaller 27 spline output yoke) after you shorten the drive shaft. The bigger stronger 4L80e will require you to also modify the transmission cross member or buy one from G-force as you can not just slide the stock one back since the TH400 was not offered as an option there is no sliding anything back.
If it is just worn out (still sort of works) I would have yours rebuilt by a professional shop (not just a national chain such as AMMCO as you pay too much for too little with them).
You are aware that there is a board that specializes in just the 1994-'96 Impala SS
and as such they are rabid fanatics that know everything their is to know about your car. You are perfectly welcome here as you meet the criteria of having a Chevy V8 powered rear wheel drive B-body car it is just that most of the members here have older carburetored two door versions of your car from the mid sixties.