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As Brian said, Group 24's won't give you that. Even a Group31 will barely be rated for 100AH, two of them makes 200AH, and of course at a 50% discharge you are back down to only 100AH useable again.
I'd also suggest one house bank, as big as you can, plus just one starting battery, big not needed. Your house bank can be 2x6V or 2/12, really doesn't matter, but if you measure carefully, sometimes you can fit a less common shape to get higher capacity. If you go to a battery distributor most do sell retail, and they'll be able to suggest the most AH to fit in the space, not just the common sizes that a retail store might have.
The external regulator should make a big difference in charging, but basically no matter how much battery you install, all that is going to happen is that they will "suck harder and longer" on the alternator. The alternator will run hotter but again...a 500AH bank at 50% will consume 250AH (plus overhead) and you can certainly fit a 75A to 100A alternator in most installations, so it would just take 2- hours to do a full 50% charge.
The thing is--as you probably know--the more battery you have, the less deeply you have to cycle it, and the longer it will last. So you do win, even if you have excess capacity. [read: safety margin against discharges]