That would be a pretty sweet set for Shuckle. It's already pretty tough to take down, and that would make it even more so. It's not that great when it comes to offenses, though, so that would make up for that and have the Pokémon seem not-so-overpowered.
As for the evolution itself, I'd like if two-fifth of its HP stat were cut and shared among Defense and Special Defense equally. This way, it would be able to endure an additional hit or two, hopefully. Design-wise, I'm not exactly sure how it could possibly look. Maybe it'd be able to have a bigger shell? This could help explain its defensive boost perfectly, if you ask me.