it might be in your best intrest to build it when you get the gun so you can fit it as you go...rather then doing it now and it not fitting.
Personally, I like the looks of the carbine stock, but mini-batterys dont have enough guts for me. If it was me, I would rig up a battery sling
http://www.arniesairsoft.co.uk/?filn...ing_review.htm
IMHO, large battery is the only way to go....but thats just me