Hi dom
Bolivia is the only country in SA I've not fished. I do know that the peacocks in the upper regions of the amazon are generally pretty small and it's not too easy to get in to certain areas of Bolivia. If I were to go there it would be for golden dorado.
I'd suggest you check into colombia. Big peacocks in the same rivers as very large payara plus a bewildering assortment of other species as well, without the insanely high prices they now charge Americans in Brazil, plus it's closer and has better flight options.
I suggest you contact my fishing buddy in Colombia, Alejandro Linares who has experience both in Bolivia, and can set you up in Colombia if you wish.
His email is:
pescaenlaselva@hotmail.comAnother option you might try is Suriname, which is one of my favorite places, but you won't find payara there or the really giant peacocks. You can catch record size butterfly peacocks in the low teens, giant wolfish (more fun than pcocks or payara) plus lots of other species also. Best part is the virgin jungle and totally beautiful rivers.
best,
L