As the OP said, this is the best way to book seats on LH web, once you finish selecting flights, and before paymant page.
But be careful, it is still a long time ahead, LH as others, can change equipment several times, and it is not 100% that this flight will be operated by 748, it can be 333 or even 380. If you do buy ticket now, go back to "manage my booking" every now and then in order to follow up the seats.