The guides will usually tell you, if you want big fish, you have to chuck them something big....all of our large fish came on big, deep-swimming streamers and buggers with rubber legs...always rubber legs. The sculpin, zonker, matuka, bunny, muddler...all cone head. We fished a 30 ft, deep sink tip as we were after the large migratory browns at the opening of the season in November. Black and olive colors seemed to do best...though each river seems to have its own character. At one point, my guide said tie on a white bugger...we promptly caught a few rainbows, then, never used the white bugger again. Go figure.
The fish there are fat and strong. You will be amazed by their strength and weight versus length. Take LOTS of sunscreen and be generous. My arm nearly fell off after 6 days of chucking large streamers at the banks. Many, many of the fish came off strikes within inches of the bank or in between two willows that over hung the banks....tough, technical fishing. Fast water, fast stripping...slow water, slow stripping.
We did zero dry fly fishing...I bet it is boiling down there now. But, the big ones are down below.
Do you have a guide? If not, take Outfitter Patagonia's number or PM me and I can send you other details. These guys are incredibly knowledgeable and friendly.