Note: I'm not an aviator, so this is to be taken with a "I'm reading things". The great circle route is rather direct -
http://www.gcmap.com/map?P=YQM-YYT-SMA-DKR&MS=wls&MR=600&MX=... (from
http://www.gcmap.com/mapui?P=YQM-YYT-SMA-DKR&MS=wls&DU=mi ) with legs of 1581 and 1602 miles.
I'm guessing weather is a key question there. This is based on https://www.airlinepilotforums.com/hangar-talk/34838-help-ne... which is taking a C172 from Maine to India and has the same question: St. Johns to Azores or Greenland and Iceland. The general suggestion there appears to be the Greenland and Iceland route.
There's also http://220kts.com/ferry-flights/atlantic-ferry-routes.html which looks at some of the options too. That seems to be a popular route: https://www.k-aircraft.de/en/news-neuigkeit/north-atlantic-f.... Note that first link mentions "good alternative during wintertime". The ferry was done in October.
Consider things like: GPS was new. The pilot was flying the "same" route as the airlines (this was important when the radio broke - if he was flying a different route he wouldn't be able to transmit until close range with VHF)
All that said, this appears to be his preferred route: http://www.aerocompinc.com/airplanes/CA6/africa.htm .
As a not an aviator, I'll guess fuel costs (Greenland is a ways north if you're trying to go to Africa) and icing concerns?
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Tangent to this... The 717s that Hawaiian Airlines flies has a range of 2,371 miles. The distance from SFO to Hawaii is 2,339 miles. To get the proper range - the inside of the plane: http://www.airliners.net/photo/Hawaiian-Air/Boeing-717-22A/1...