the SS frame and the BH frame are NOT the same, the SS frame is smaller
Since 72/73 the SS(single six) and the BH (blackhawks) have used the XRN-3RED frame for the New Models. These replaced the XR3-RED frames.
http://www.gunblast.com/Hamm_Ruger-SA-GripFrames.htm. The only frame differences I'm aware of is the changes for the infernal lock. Below are the NM BH .357, Super Single Six and NM BH .45 Colt. Frames are the same. Grips are interchangeable, even on the .45 with the infernal lock. If you want a single six in .357, it's called the NM Blackhawk .357.
I did a bit more reading. Ruger parts identify the
grip frame and
cylinder frame as separate entities. My Single Six manual has a part number for the cylinder frame. The Blackhawk manual mentions cylinder frame, but doesn't have a part number.
So, if the different size "frames" are "cylinder" frames, yes the SS frame is smaller that the BH frame. However, to convert a SS cylinder frame to take .357, you'd have to lengthen it and beef it up. I could probably do that by putting the .357 cylinder frame on the SS grip frame. In other words, if you want a SS in .357, buy a .357 BH.