I'd like to add that personally, I would avoid the UPS batteries (unless they were free and/or never used) as UPS systems are notoriously designed with the batteries being the "weakest" components. ie, a UPS typically draws several C, a scenario which is just not conducive to healthy batteries. They tolerate it a few times, but give out rather easily in their intended environment. Once a UPS is 'done' with them, they're not good for much else either.
As bad off as you might end up with the forklift cells, even with sulfation and all, at least they were designed with the idea that they would be delivering their charge over the course of several hours, not minutes. This is a much better scenario IMHO, and would lend to healthier cells in general. Although
Besides, even if they're only capable of delivering 50% of their original design spec, you're still sitting at a pretty 300AH; the (likely sickly) UPS batteries you're describing would only be good for 1/10th that (maybe) if they've seen life as originally intended.
I'd be willing to bet that desulfating a 600AH battery would be a skull numbing concept at best, but worth it if it paid off.
Just something to keep in mind.
Steve