Just connect the overflow that used to be on the hard tank to the bag. You will have to connect a longer hose. The one off the hard tank is pretty short.
For what it's worth, unless you plan to load the boat all the time, I would stay away from the 750's and go with the 400's, less hassle than trying to get the same amount on each side. I have been running the 7's and am going to the 4's. I will keep a 7 onboard to throw in the locker for surfing.