Direct swap, just better parts!
Why not go with a 4l80?
Doesn't that involve a crossmember mod? or is the 4L80E also a direct swap?
No, The 4L80E is not a direct swap. The cross members are different and the driveshaft will need to be shortened as the trans is longer.
There is a thread here somewhere where someone swapped out his 4L60E for a 4L80E. - fairly involved.