:I have no idea, just from looking at a map, I don't think they are linked hydrologically, but looks like there are several places where a portage from one basin to the other wouldn't have been hard. Though crossing from the Parana's headwaters would lead to the Araguaia or Tocantins which would lead to the Amazon delta rather than the Orinoco.  If you took the Parana to the Paraguay river to the Pantanal region - it's headwaters are very close to both the Tapajos (via Juruena or Teles Peres) and the Madeira (via the Guapore), the Madeira-Guapore would probably by easier to navigate in terms of the number of waterfalls you'd need to get past.