I also have 4" SP 101s, 32 and 22, and looked into the conversion at Bowen Custom Arms. The man hisself replied to my email and said can do, $100, ship the cylinder on the crane. No expensive overnight shipping for a gun; its just a part(s). Said turnaround was fast but I can't remember the specifics of how fast.
Side issue; I have adj. sights on both my 4" SP 101s, and I think they are great that way. A 327 set up that way would make lots of sense. Not knocking the GP 100, but the SP 101 seems like an ideal platform for a trail-farm-utility gun. But windage-only adjustable sights? What is the logic behind that decision? Factory 22s and the range of hand load and factory fodder for the 32s beg for fully adjustable sights.
Essay W.