The 6L80 is about the same length as a 700R4. Your problem will be the girth of the transmission. You most likely will have to modify the transmission tunnel to some degree. They do fit T56 transmissions in there with no problem though. So you may not have to modify the tunnel to much. Also I don't know how much ground clearance you will have. You might have to use the G8 transmission pan.
The other problem will be mating the new ECM and wiring harness with the existing wiring harness. It can be done if you have experience in that field.