Don't know about the whole seat selection thing. As Westjet is currently wet-leasing a 757 from North American Airlines to fly the routes from YYC and YEG.
North American Airlines does not show up on the seatguru website. I know for sure that they are 3x3 for seating and that with 757's there is usually a hella sweet emergency exit row with lots of leg room around the middle of the plane. Try and get that if you can.
It should be around where the 2nd door is from the front in this picture of the plane:
WestJet 757!!! by
Wee in YYC, on Flickr