I am having a hard time understanding what you are trying to say...the stock flange is 27 spline and cannot be reused with a Nitro. Nitro's are all going to be 29 spline, whereas Yukons and presumably G2's are 27spl, in which case you can reuse the stock flange.
Nitro's and most other reboxes are manufactured in South Korea, they all set up pretty well and are pretty quiet. I am not sure where the Yukons are made, but it is a Randy's product so likely SK as well, also note that USA Standards are a Randy's product as well, just lower quality.
IMO you can't go wrong with either, but the pinion is larger on the 29 spline model, even though it isn't a normal failure point, why not upgrade.