Could a very long password theoretically eliminate the need for a slow hash? Before I provide details, I want to clarify that I am not looking to implement this practically, but I'm only asking to get a better understanding.

The way I currently understand it, we use slow hashes for passwords because there are too few possibilities in passwords. This means people could calculate all the potential passwords (or a whole lot of them) fairly quickly when using a performance hash, and find what made the digest. E.g. if in an imaginary world where digests are 3 bytes and slow hashes last a decade my password is "abb" and it results in digest "x3x", someone who got this hash could start retrieving my password by hashing "aaa", then "aab", then "abb" and see that the result is "x3x", and so my password must be "abb". With a slow hash, he wouldn't have time to calculate all three results.

This makes sense for short passwords, but I thought the strength in modern block ciphers lied in the key space. For Two-fish for example, it can have a key from any of 2^256 different variations. Trying all those variations would just be insane, and so it can't be done (for now).

Given these two things, wouldn't the strength of a performance hash as a password storage tool increase with each letter? Alphanumeric letters cover 32 to 127 in ASCII, so that's 6 bits entropy per character (I think). If that's true, would a password of 43 characters be secure when hashed with say, SHA 256?
Despite the impracticalities of using a 43-char password, I would say yes, such a long password would be secure when hashed with just SHA256.  Assuming 127 possible ascii characters, a password of 8 characters would require an attacker to search about 2^56 possibilites (viable), whereas a password of 43 characters would require searching about 2^300 possibilities (infeasible, considering that 2^128 for encryption using a 128-bit key is infeasible). Having such a long password would make a slow-hash redundant.  With the case of a 43-character password, I doubt that rainbow tables would be issue either.