[79 silverado]

With the info you have here it is difficult to answer precisely... but here is what my thoughts on the matter are and you can fill in the blanks.

There are two kinds of lift kits:

1) Suspension lift in which shackles/springs/etc are changed out. This kind of lift also changes the angle which the driveshaft runs at, and may require a lengthened driveshaft to cover the increased distance from the output to the differential. If you are changing out the driveshafts later there is the chance of messing something up ad creating a leak at one of these output shafts in the future if you are not careful.

2) Body lift in which the factory body mount bushings are replaced with spacers/blocks to lift the body higher off the frame, but leave the chassis and running gear unaffected in their stock configurations.

Sometimes someone uses a combination of a couple inches frame, couple inches body.

If your lift is purely a body lift kit, then I would go ahead and change out your transmission first, improve driveability and reliability, and know that the chassis and running gear do not need to be touched when you alter the lift kit height.

If your lift is a combination of body and suspension, and the way you want to change your ride height is to remove only the body lift and keep the suspension lift kit in place, then same answer as above.

If your lift is a suspension lift only and you are going to be removing this suspension lift and changing around the other driveline parts I would probably just do everything at once later.

The other thing I should add is that the crossmember and mounting point of the transmission unit itself remains the same with respect to the frame rails in any scenario unless you are putting in an aftermarket crossmemeber or anything like that.