Let's assume I have a strong enough antenna at exactly the same frequency as the victim. If it sends a request over unprotected http to the router, could I then listen to that request and respond to it before the router does? Would I need the MAC address of the victim too?
HTTP does not protect against this whatsoever. So that question is then pushed down to the type of wireless connection.
For example a private wifi uses a shared secret (password). If you don't know this password, you should not be able to neither read, nor change/respond. (Though vulnerabilities in wifi are not that uncommon).
If the wifi is enterprise, you would not be able to do this even if you had login credentials, unless you had the private key of the router.
If it is an open wifi, then yes, you should be able to do it quite easily.
And of course, there are meny other possible connections and other factors. For example a VPN would prevent this, IPsec or IPv6 encryption would prevent this etc. You need to be able to break or bypass all levels of security to intercept the connection. HTTPS is only the one that is always talked about, because it is the most encompassing, available and reliable of them.
You can set up your own compromised AP that poses as the original AP, often called Evil Twin. If you can make the victim connect to yours instead of the real one, maybe by DoS'ing the legitimate access point (AP), then you will have control over any traffic the victim produces. You can see this in action in this video by hak5.