Well, you guys got me wanting to read the book again, so I checked it out yesterday. I heard a rumor that one of his sources was actually from the Weekly World News (a US rag that makes it a point of printing nonsense), so I'm checking the bibliography to see if there's any truth to it. If it's true, it would cast serious doubt on the author's research skills (not that they were perfect to begin with).
About the giant woman and baby, I'm not sure if it's in Forbidden Archaeology or not, but I know that she's mentioned (briefly) in Lost Cities of North and South America. There's also a photograph of the mummified pair. However, she wasn't found in the Grand Canyon, and she wasn't spectacularly tall, only 6 foot 8.
The caption reads: "Researchers Brad Steiger and Ron Calais report in their book 'Mysteries of Time and Space' that in 1895, a party of miners working near Bridlevale Falls, California, found the tomb of a woman whose skeletal remains indicated that she had stood six feet, eight inches in height. The corpse had been wrapped in animal skins and covered with a fine gray powder. She was clutching a child to her breast."
You're probably getting it mixed up with the old Arizona Gazette article about tombs in the Grand Canyon.