A lot of people just avoid the 8.1 because they aren’t easy to find replacements. Aftermarket parts are expensive and they just want the easy way out. If a person looks for a while they can find good 8.1 engine and Allison 2wd trans. I Kmow a guy who has a bunch but it’s all 4x4
From the bit of research I've done on the 8.1, they are heavier and have some oil issues, the earlier ones anyway. The consensus seems like if you don't need to tow anything, and I don't other than a$$, the 6.0 is the better choice as it's lighter and can get more power easier.
The LS swaps are figured out pretty well by now, easier to get parts and info of anything is holding you up.
And ya....you know as well as I do, up here anything 2wd is pretty rare unless it's a van or some sort of fleet rig that's probably been beat on pretty hard.
Not sure I know enough about the Allison trannies that I'd wanna mess around with them. I believe they are bigger than even the 4L80'S and I don't feel like hacking up my floor.
But maybe with some more research I'm sure the 8.1 will take a 4l80 and maybe have to get you to reach out to your source and see what a 8.1 goes for up in these parts. I tried Kijiji and could only find full vehicles for sale or generic "we sell engine" ads.
Edit.....the only reason I'd do a 8.1 is cuz I love to be different. If my boy still had his old truck, a 5.7 Hemi swap would still be on the table!! Lol