"Whoever you are, you had better not talk to him like that" This one is fine.
"Whoever you are had better not talk to him like that" Technically probably correct, but not at all a natural things to say.
It is saying that the person you are had better not ....
This is rather a strange way to say "you should not ...."
The essensce of the sentence is that it does not matter who you rae, but it says that the one who you are had bettter not...
my point? The sentence tresses that which it implies is irrelevant, hence not at all a natural thing to say.
Whoever you are had better be a captain ablt to fly this plane. - almost plausible.
"Whatever gift you got me had better be good" Ok
"Whatever gift you got me, it had better be good" also ok
Hope it helps.