They look like pretty cool little kits. I got two 10/22s dressed up like ARs and would love another dressed up like an AK74u.
Also, barrel length doesn’t matter as it’s a rimfire, so long as the overall length is 26”. So the 16” barrel would be just fine I imagine, probably even the 8.5”. Personally, I’d go for the 8.5”. Looks a lot slicker.