When you want to do a 4L60e to 4L80e Swap, you will either have to make or buy the Transmission Cross-member. I decided to make mine. Given that the 4L60E has a length of 23.5″, while the 4L80e has a length of 26.4″ the cross-member has to be relocated, or modified to work with the new 4L80e.
I welded a 1/4 piece of angle iron to the existing cross-member to extend it back the 2-2.5 inches required to accept the bolts from the new transmission (it’s actually the transfer case adapter that bolts to the cross member). In addition, I drilled two holes in the angle iron and bolted them to the cross member with Grade 8 bolts. Although my welding skills are beginner at best, I don’t think its going anywhere.