Sweat soldering with soft solder is the way to go. It will not hurt the bluing or ruin the heat treatment of the receiver. It is also safe to solder the bases onto the receiver with the scope rings attached. Note that the european craftsmen that do these mounts always use soft solder.
Brownell's Force 44 solder is what you can use, although a basic 50/50 tin/lead alloy or even an eutectic (low temperature) 63/37 electronic solder is suitable. The soft solders melt at about 600 degrees Fahrenheit (less than 400 degrees for the eutectic solder) with more than adequate holding strength.
The key to getting a good, strong joint is to carefully fit the mount bases to the reciever for 100% contact. Scrap, file, and try for a close fit with no rocking motions indicating surface mismatch. Mark, scrap, and file the receiver surfaces to remove all bluing. Tin both surfaces if using ordinary solder and rosin flux. Acid flux will damage bluing. Apply the sheet solder and flux if using the gunsmithing solder. Clamp into position using bare wire twisted up tight. Apply heat gently using propane or soldering torch. When the solder melts, tighten clamp and hold until cool. Use brass brush and alcohol to clean off excess flux. Done properly, the soldered joint will be almost invisible.
