Speaker wire is going to be too thin. After you read the above threads and you've learned a bit more then I'd personally go with 8ga to 12ga. To repeat what Red said "NEVER skimp on power wire, NEVER."
Also, I imagine that you will be needing more than six feet of cable once you get to going around and over stuff, through the firewall and wherever. Length is another thing not to underestimate on. If it's too long, you can cut some off but you'd can add to it... (well, you can but it's more trouble).
Lastly, I would recommend not using the clamps. You'd risk the clamps popping off and shorting out your system or perhaps even starting a fire. Either get a multi terminal post (you can find these at WalMart for $5/ea), cut the clamps and direct wire or cut the clamps and add terminal rings.

(These are just examples of the connectors...)
Derek