8H and 8J are blocked for airport assignment. They never "become available", they go from blocked to assigned (or they go out empty). If you want them, get to the airport early and ask the TA or GA to assign them to you--if they're still available, they're yours. Of course, if you have a disability that somehow means you need to be near the loo or for some other reason need those seats, a call to AA's disability services folks would get those seats assigned to you in advance.

All seats have exactly the same amount of legroom. This is because the footwell/IFE insert and the seat itself are a unit, so even bulkheads have the same footwell space as non-bulkheads. I found two a.net pictures (here and here) that should sort of help demonstrate what I mean. I would suggest booking seats on the starboard side near the back if you're a couple travelling together, as you'll have less foot traffic and first choice of meals. If literally all seats are available, choose 12H and 12J. If OTOH you're travelling alone, I'd pick a centre section aisle, as there's less chance your seatmate will need to climb over you if you're sleeping (the middle seats are also the last to go, usually). On this flight, again assuming all seats are empty, I'd choose 12G.