There are very good instructions for installing a thermo in the lid listed in the WSM Mods forum on this site. I did this to my WSM and it worked great.
Rib roasts do not need "low and slow". 350-375 degrees for 2-2.5 hours, perfect. I use 2 parts oak to one part hickory. Works perfect. Cook to an internal temp of 125 degrees and let rest for 30 minutes. Yummmm!