Craig,
Follow quicks advise on the plug, and yes the Williams WGRS will work on the Huntsman and Sidekick. The sighting radius is a little different on the 24" vs the 26" barrels, and you will generally be in the top half of the scale on the side of the sight. It will screw right into the two back holes for the scope rail.
IIRC the Williams part # for this sight is "WGRS-HR". I bought mine 8 years ago for a Colorado elk hunt, but at that time it was much cheaper to go through the H&R/NEF Customer Service than Williams or other places. They stocked this sight for the BC standard sight. At that time the invoice listed all parts separately, so make sure whoever you talk to knows you want the whole sight. I haven't dealt much with CS since Remington took over, so I'm not even sure if they carry these for sale anymore.
These are good shooting rifles, excellent to outstanding once you get the right components. Good luck!