Agree that cloyster/blastoise is a much better choice as a shell smash user, I doubt Gorebyss is really OU viable tbh, but if you still want to use him you can try.
Honchkrow though is tricky to use because of his frailty and bad speed tier but still is certainly usable as a wallbreaker, I don't think it's necessary to change him for Mandibuzz especially if your team is more offensively-oriented, though there are admittedly "superior" choices.
Ferrothorn is a great pokemon that fits on any team, though a more specially defensive EV spread w/ leftovers is better IMO because of the overabundance of fighting-type moves, as well as the fact that most of the moves it tanks are special.
Cofagrigus is a fine choice as a physical wall, though I don't think he should have nasty plot, that's more for the trick room set which doesn't really fit on your team. Maybe gliscor could work better here, it offers an electric immunity, a fighting resist and defog, access to taunt also helps your matchup against more defensive teams.
Of course there's still a weakness to hydreigon that needs to be patched over. There's quite a few options here. I've had a lot of luck with max sp. def rotom-wash which can actually tank draco meteors quite well, has access to recovery with pain split and is just an extremely good pokemon in general. It also has defog if you decide against gliscor, or use a different move on it instead. Other possible defensive checks include tyranitar, tentacruel, milotic, gastrodon, and togekiss. Of course there's always the blob sisters (blissey/chansey) as well but they're rather dull.
Alternatively, you could try to check it offensively. Not a ton of options if you're trying to outspeed most common Hydreigon set which is choice scarf but there's conkeldurr (mach punch), scarf infernape, dragon dance salamence, your own scarf hydreigon, maybe even something like scarf mienshao.
As for dedicated special attackers, it would probably be best to include at least one--it's up to you what you would replace.
Hope this helps