What are some of the 6 Speed Automatic Transmissions in GM vehicles that are known to perform well and are considered to be durable?

I'm wanting to explore the possibility of installing a 6 Speed Automatic in a 2009 Chevrolet Colorado that came from the factory with a 5.3L V8 fitted with a 4 speed automatic.

I understand there would be the need for re-programming the ECM...but, first off I'm more interested in the fitment of doing a swap like this.

Do you think it would fit physically without having to alter the cab's floorboard sheet metal? If there is a difference in drive shaft length, we have a commercial drive shaft shop that isn't far from me so that shouldn't be a big issue.

Just curious if anyone had experience / thoughts with a swap like this...