Can I use a Scosche 1200 Watt Amp Wiring Kit for a 600 Watt Amplifier?
Amp wire can only be too small, never too large. The only drawback would be if it's unnecessarily large making it a hassle to work with. From what I can tell, the 680w kit is 8 awg wire and the 1200 kit is 6 awg so there's not a lot of difference.
That would be OK.