Foremost among the seven hydro-electric wonders of the world is Canada's gigantic power project - Manicouagan damsite No. 5 - now rising up in awesome majesty deep in the rugged scenery of wilderness Quebec. Part of a multi-dam engineering feat to harness more than seven million horsepower, this modern concrete colossus will have a towering central arch big enough to shelter Montreal's 600-foot-high Place Ville Marie skyscraper. Already, with diversion tunnels closed, this man-made complex massif, welded with engineering mastery to the sub-surface bedrock is taking the first strain of what eventually will become an 800-square-mile inland sea upstream. As a continuous procession of 22-ton buckets of concrete passes high overhead on cableways and 3,500 men toil in concerted effort, the 4,000-foot-long, 703-foot-high dam grows apace to meet its 1969 completion date. At that time Canada will be richer by another 5,000,000 kilowatts of vital power - made possible by the technology of Quebec's skillful hydro-engineers.