AA doesn't offer First Class anymore between NYC and Rio these days. All their 777-200s - which is what they use on this route in addition to the 767s - have been converted into a two-class configuration without F.
In order to get AA three-class F on the longhaul, you need to book on the 77W (the only AA aircraft retaining F). GRU is the only South American destination that gets a 77W - one daily each from DFW, JFK and MIA (flight numbers 963, 951, 995). This turns your trip into a two-stop itinerary BOS-JFK/DFW/MIA-GRU-RIO, with the intra-Brazilian flight likely on an economy-only narrowbody (Latam has one daily 767 flight with business class, though) and up-to-4.5-hour domestic feeder flights in 737s. Not very premium.

It's the same with UA, btw. No more F to Rio. Only Brazilian destination that still has GF is GRU.

The only oneworld option with F all the way would be taking BA via LHR.

I'd give up on F on that route and rather fly business via JFK. Given the choice between a Latam and AA 767, I'd prefer AA who at least offer all-aisle access and more privacy with their staggered 1-2-1 seating, whereas you'll have seatmates with Latams' 2-2-2 seating. Seats are equally narrow, though.