I'd also check the flight loadings for your specific flight if I were you (if you don't have an expertflyer account a friendly soul here will probably help you). Last time I sat in WT on a 747 I enquired about a paid upgrade when I got to LHR and the very friendly check-in lady said that she could happily sell me one, but was I aware that there were only single digit numbers of people checked in to WT on that flight? (can't remember why it was so empty). So instead, she simply asked me which entire empty row I would like to sit in. And then the cabin crew (who basically had almost no service to do) proceeded to feed me more food and drink than I normally get sitting up the front.

So the above advice about the seats is all very sensible, but worth keeping half an eye on the flight also. Not that my experience is anything other than extremely uncommon.

HTH