Have Brazil occupy the 3 guainas when Europeans are locked in struggle (Napoleonic Wars? WWI? And, probably at a different time conquer Argentina (taking Uruguay en passant, and Paraguay and Bolivia see the writing on the wall and agree to annexation as autonomous republics).
Early on, most communication was by sea, and most settlement was near the sea. This alt-Brazil would have to be (or become) a significant maritime power.
It would help if they allied with the US to keep revanchist European powers from taking back their Guaina colonies.