[SBTork]

Awesome plan for an LS, that'll make this rig the perfect driver!

As far as the 4L80E, I've looked into this and don't know enough to solve the issue but I can possibly add to the confusion, lol! I'm doing an 4.8 LS swap on my '83 K5 and was looking into a 4L80E and got some mixed info too and I decided to stick with a 4L60E. That and because mine will be a driver/occasional wheeler so I don't need the benefits of the 4L80E, even though I like the idea of having an overbuilt transmission. Anyways, I emailed Advance Adapters about this such situation and here's the reply I got, maybe it will help....
"Adam, Thanks for the inquiry. The 4l80e will bolt up to the NP208. The spline count on the NP208 has to be 32 spline. If you are replacing a 700r4, you will have to change the input shaft. Th400 and SM465 transmissions had the correct 32 spline count. We do not manufacture the NP208 input shafts. You will also need to make sure the rear sensor in the transmission has the reluctor gear. The 1997 & newer 4WD 4L80E transmissions have a rear sensor provision; however, the reluctor ring in the transmission is left out since they ran it out of the transfer case."

So if you get a '97 and up 4L80E it will have the provision for the reluctor ring in the transmission, but it will have to be added. It sounds like that along with changing the input shaft on the NP208 to a 32 spline unit should be enough to be ok, but maybe someone that's done it on here can shed some more light on it too.