Pardon for blowing your bubble but sniffing ssh keyexchange does not do you any good. The symmetric key is exchanged via a channel aready secured. The keys that is used to secure the channel used to exchange the symmetric key are exchanged via DH-based protocol. If you want to spend your time factoring primes for next 500 years to extract the key, you are more than welcome to try. It is crypto-101.
If you can arp spoof as indicated in the message you are replying to, you can perform a MTM attack which SSH offers only minimal security against (in the form of stored host keys that users often choose to ignore or not verify the fingerprint). Look to SRP for a MTM-less password authentication solution.
Monkey in the Middle attack on SSH is very difficult to perform. I'm cc'ing Matt Bishop (bishop@cs.ucdavis.edu) who together with yours truly wrote a paper on this in 1997. Cheers, ALex