If you HAD to do coach, I'd definitely go for the bulkhead exit row (31??) option on the 777 over an exit row on the 763 any day. I've done both, and the 777 beats the 763 (bulkhead exit row/coach) hands down any day.
I'd recommend snagging an Ambien, valium, whatever to help sleep thru the flight. IIRC, you (the OP) are a Plat, so you'll have AAdmirals Club access on the day of your flights--the drink coupons could come in handy stateside, as would the open/free bar at the EZE A/C.

Spending 50K to upgrade to business is a great idea, avoiding the $250 copay by a nice exchange over coupon connection for a couple of eVIPs. Just make sure your fare is an upgrade-able fare (NOT Q, for example).