Gas springs are pretty easy. Just play with the location a bit to see what will work for you... Having the spring on the more "out of the way" side, placement of the spring to allow the hatch to open far enough for your liking, etc.
All springs will have specs that show, in addition to the poundage, the extended and closed lengths. Mounts can be flat or angle. You can create a more solid area to attach the mount (if you prefer not to thru-bolt) by fiberglassing/epoxying a wood block to the back side of the hatch, for example. You could even paint the block after installing so it looks good.
All in all, pretty straight forward and not very hard. Something in the 20lb or 40lb range will be fine. DO NOT buy automotive spring lifts. Spend the couple extra bucks on a marine lift (Attwood or Sierra, for example) and they'll last much longer. SS is certainly better than the standard marine lifts, but that's your call.