Now this a swap i would like to see happen. I mean the 4L80E is better than the 4L60E but the Allison is a different animal. Having driven trucks with Allison and 4L8OE tranny’s I would definitely go with the Allison. Not only is more durable but is 5 spd auto and could tow a lot more.Shouldn't be a hard swap, even if the bellhousing doesn't match you should be able to swap that. But you would still need to either have the ECU updated, or replaced.
Why not see if you can get your hands on a wrecked 4x4 with an allison tranny.