Here's a map of St Johns found by the guys on newfoundlandmotorsports.com.
Also on the subject of Canada, I see no ... St Johns (Newfoundland) used for an Atlantic race in July 1974. All I know about St Johns is that the Montreal Gazette said that it would "take place on highways around the provincial legislature in St Johns after the pattern of Trois Rivieres". The edition of the Gazette that would have covered the race and might have given a few more clues is missing from Google's archive and I haven't yet acquired any documentation from the race.
I've had a go at working out where the track was but something is wrong because there is absolutely no sign of the southernmost corner of the track on the ground. It's hard to believe that they dug up the track without leaving any trace. Maybe somebody can do a better job.
Google Map of circuit