I chose the Slimline because it could lie on its side as I couldn't figure a way to put the capacity I wanted in or around the original battery box. I wanted to slide them under the aft part of the quarterberths, into the short space back there that's not good for much else. It's not very ideal but my Beta engine weighs significantly less than the J/30's original Yanmar, so the extra weight isn't too concerning to me.
If you wanted to put both on the same side you could (removing the original battery box), but that'd be a lot of weight off center.
I haven't done any fiberglass reinforcing, but I do have redundant straps holding the batteries in place - one across the top and another on U brackets through-bolted to the engine compartment walls.
Another option is to put the new batteries behind the engine and shaft packing - there's plenty of room in the engine compartment aft. Weight-wise it's probably not the best option, but you can certainly fit some serious capacity into that volume.
I don't know if you intend to race your J/30 one-design, but the class rules probably have something to say about all this. I don't race my boat, so I haven't really worried about that.
