I'm going to date the girl my best friend has a crush on
Do you even like the girl?
No, I don't like her at all.
Being a good friend comes with responsibilities. If you know your friend has a crush, going after that girl - especially if you are doing it out of spite - is not being a good friend.
What he needs to understand is that I have the big cajones around here.
You don't sound like much of a friend to me; if you're so desirable, you shouldn't have much trouble finding a suitable mate. Why chase his crush? How would you feel if your friend moved in on your potential girlfriend for sport?