I've known about these giant remains most of my life. I was first introduced to them by Ivan T. Sanderson's book, "Investigating the Unexplained" back in the early 70's. Living in Ohio, I'm more than familiar with the Adena and Hopewell earthworks in Newark, Chillicothe, and throughout central, southern Ohio and southern Indiana. I've visited a number of them and the existence of giants is no secret. It's just not reported in the mainstream press.
For a list of giant sightings, check this site out: http://www.freerepublic.com/focus/news/830123/posts
Another thing Sanderson mentioned were burial jars containing tiny human remains as well. Basically, the New World's version of African Pygmies. If you can find a copy of the book, you might be impressed at even more things that are in museums and are either deliberately mis-labeled or just flat out misunderstood. Lots of interesting stuff in it. And once you read it and some other "off the wall" historians, you'll have the same deep-seated distrust of museums, curators, and the Establishment that Vieira has.
For instance, here's one on ancient jet aircraft models: http://www.world-mysteries.com/sar_7.htm
I don't think everything that gets reported is credible (Check out "Our Hollow Earth" on Google), but I also have a deep skeptical distrust of institutions in general, particularly when they are tied to a political agenda. I'm probably not as jaded as Vieira, but then I don't deal with these folks directly.