An old 600 cylinder sac fits perfectly if you're looking to add it in addition to the ibs. The new 750 is humongous and will take up the entire length very far into the cockpit, not to mention put a ton of stress on the side walls of the driver console and make it flex. So a new fly high v drive sac I think would fit.
Honestly though if you can grab 200-300lbs of lead/pop bags and throw them as far forward as u can it works just as well. Even when you want it empty the lead adds to the performance amd helps the ride in rough water.
Current: 2008 Super Air 210TE
Previous : 2000 X-Star, 3000lbs ballast, LTR 330, Acme 1235 14.5X14.25prop. 1200 hrs of flawless fun