What do I need to do to swap from a 4l60 to a 4l80 trans in a 99 ss?

 

Use the Search -> Advanced Search function. Select only the "Automatic Transmission" forum and enter "4l80E" as the keyword. This will give you great threads like this one:
https://ls1tech.com/forums/automatic...en-camaro.html

I know little about 4l80e, but from what I have read here:
1. Get a 4l80e and consider getting it rebuilt with a few upgrades.
2. The tunnel of your '99 will need some mods (with a Ford wrench) to make it fit.
3. Get a good converter; budget $800 for this.
4. The wiring harness needs some mods or spend $150 for a "converter".
5. The PCM needs some mods - I don't know the details.
6. Not sure if the driveshaft needs shortening, but it is easy to find a local shop and perhaps $120.

As many others have done this, do the search, read the threads and then ask more specific questions and I am confident you will get quick answers.

 

He is right, but you will also need a crossmember. There are two, 1 with torque arm and a tourqe armless one, you will also need a th 400 slip yoke. A gm dipstick will work but you have to bend it a bit or call Jake's he offers a dipstick specifically for the f body swap. Might want to consider a cooler with braided lines and use an fittings.

As far as moding the tunnel, some cut holes, that is not needed. All you have to do is massage the driver side with a flat air hammer or a hammer to allow room for wiring harness and shifter linkage.

If your keeping the 60 flexplate make sure and tell converter company so it is built to fit.