Benefit of combining classical substitution ciphers with modern cryptography
The fact that a given cipher has a key length of 296 bits doesn't mean at all that it provides 296 bits of security or even that a brute force attack would take $2^{296}$ steps. The problem of mono-alphabetic substitution cipher is the ridiculously small block size (in this case, barely $\log 64 = 6$ bits). 

If absolutely nothing about the plaintext is known, every cipher is unbreakable, as there's no way of knowing if you got the right result.

In practice, you always know something about the plaintext. If that's not enough to launch a known-plaintext attack, your method might add some security form a theoretical point of view, but it's rather pointless in practice, since brute-forcing a 256-bit key is just as feasible as brute-forcing a 552-bit key.

However, if you do know enough plaintext, the mono-alphabetic substitution cipher doesn't add any security at all. After encrypting the known plaintext with an AES key, all you have to do is check if matching characters get mapped into matching characters. If they don't, discard the key; no further tests are required.