Don't think it worth it to try to target steelhead on the Rio
Best river for Sea Run Browns is the Rio
Grande on Tierra del Fuego with several great lodges available. Late Jan and Feb IMO are the best months.
For Dorado, have fished the Juramento out of Salta, NW Argentina. November. Have not fished the Ibera Marsh.
Fishing techniques are totally different for steelhead and sea run browns since the rivers are big, the wind blows (really blows) and two handers work best to handle the Rio
Big Boy 24' 100, 200, 300 and gulp, 400 sink tips.
For the Juramento, it is high speed nymphing since the raft is going by at 3 to 5 mph and you are casting to pockets 2 feet from the bank and casting 5" flies. (lost over 50 flies in 6 days of fishing, but nailed a 24#). The Sage bigmouth rod was really good. From my friends, who have fished the marsh, fishing is more stealth with long, accurate casts working best.
Good luck and a fun, fun place to fish with great people...