I'd prefer the 747-400 with 2 hour connection if it was me. Just gives you a good buffer. I'm usually going through immigration so I'm not sure how long it takes to change flights in Narita, but given you are going onto another Intl flight, you should give yourself that extra time. What if your flight is even 20 minutes late out of MSP, etc? If you will be in Business class, try for the upper deck, it's quieter and seems to be it's own little world. Row 73 a/b are premo, if you can get them and 77 looks good as well.
If economy, maybe you can try for Exit row 35, extra leg room, and you can always use a carry on as a foot rest to relieve that nagging back strain that comes from sitting in economy on long flights.
Good luck and enjoy your visit to Guangzhou.