Slothead....I'll add a few comments here but may not fully answer your questions.
In the early 1970s at Watkins Glen, a World Championship 6 hour race for sports prototypes was held on the same weekend that a ~2 hour Can Am event took place, 6 hour enduro on Saturday followed by the Can Am on Sunday. The Can Am rules were few, basically a car with wheels covered by fenders and two seats; no limit to engine displacement. World endurance sports cars had much more stringent rules and, in 1970 and 1971, a 5 L limit to engine displacement.
In 1970 and 1971, the Porsche 917s and 512s were competitive with the Can Am cars and successfully ran both races at Watkins Glen. Listings of those results can be found on line. Here are the 1970 results:
http://www.racingsportscars.com/results ... 7-12c.htmlBy 1972 the endurance sports cars were regulated down to a 3 L engine limitation and could no longer compete with the much more powerful Can Am cars.