I'm working on a similar project... SPR Mk12 Mod0
I'm using a G&P full stock simply because it was available right away. Here's what the arse end looks like:
It fits my 8.4V 3600mAh no problem and could probably fit another cell easily. I rather like that adjustable sniper stock you linked to. Has the benefit of large capacity and adjustable length if you have longer monkey arms like me... I would stay away from crane or other similar adjustable stocks (despite how sexy they look) simply because you have to buy crane stock batteries; and the extra wear and tear of sliding the stock could cause a wire short.
But hey, this is airsoft, so you have to pick something you like and run with it.
BTW, Here's Illusions build of the same rifle...
http://www.airsoftcanada.com/showthr...&highlight=spr