LSX going from NV4500 to 4L80E

[Accelo]

You didn't mention the year of the 5.3L.
Copied from the lsx site.;

The basic breakdown for the 4L80E goes like this. 91-93 is the first style, 94 is a one off year and got the improved plug in it as well as the PWM solonoid for the TCC was changed to eliminate the cleaning cycle that earlier models had and the 4X4 models no longer had a tone wheel on the output shaft for the PCM as it used the VSS in the transfer case to do all of the work(some 94's still had a tone wheel in certain 4X4 applications). 95-96 is another style, same basic trans as the 94, but it got a boss on the case to mount the NSBU switch(neutral safety back-up switch). 97-early 99 got some improvements in them internally and had the trans cooler lines enlarged and they became known as a rear oiler as the return line was moved furthur back on the trans case. Mid year 99 the 4L80E received MANY improvements internally which included a new input shaft, wider band, and some oiling changes internally. The 4L80E was used on up into around 05 or so and was badged as the MT1 option whereas the 4L85E MN8 option was used primarily in motorhomes and the 3500HD chassis and other HD applications from about 99 or so. Some models were said to have straight cut planetary gears in some portions, solid input shafts, and the 5 pinion planetarys. I don't know all of this info to be 100%, but it is what I have found from alot of online researching and my ATSG manuals. You can take most any of the 4L80E's from a newer and swap into an older model as there are adapters for the cooler lines to facilitate this. And the PWM solonoid can be swapped from a 91-93 into a newer model and allow the use of most any of the 94+ 4L80E/4L85E into a 91-93 truck.

You will have to add the plug and wiring for the 4L80E to the harness and then have the ECU reflashed. Without the years and computer type it is difficult to answer the question exactly.