First, before I can give you a definite answer on the iMac mount, I need to know which model you own--like when you purchased it (its model year designation) and/or the model number. As you may know, iMac has undergone pretty major changes in the last year (now they even have a VESA-ready version) and it makes a world of difference if, for example, you have a 27"iMac from mid 2011 versus an iMac from late 2012.
Your choice of the 200 Series Pivot with a Wall Plate or Wall Track is a good one. The plate/track should distribute the weight adequately. Use toggle-anchor bolts to fasten to wall; with the standard Wall Plate
, you'll need to make a trip to the hardware store for those fasteners, or get a Wall Plate Kit
with toggle-bolts included. A Wall Track
covers a larger surface area, so potentially it will give even more support (need to use your own fasteners).
Attach the Pivot to the wall first via plate or track, then get another person to help you hold the display (just to ensure there isn't accidental display destruction happening). There will be plenty of room to get your hands behind the display. The pivot comes with thumb-screws that make attaching the display to the pivot's VESA plate very easy--no tools are required, you just hand-tighten and you're good to go.
Hope that helps. Let me know about the iMac version you're mounting.