You could always do the M4 stock mod on a K, then use a crane stock to put a large battery in...just run the connector wire through the current battery well in the body, back to the crane stock battery compartment through the stock tube.
Bingo, v.3 mechbox with a large battery.
Edit: And now, a shameless plug...
http://www.airsoftcanada.com/showthread.php?t=12053