Good job, where did the donor part come from ? Also, soft-soldered ?
Yes, agreed - given in this instance, how the surfaces orient towards one another...no 'usual' forces applied to that touch will likely make that solder joint fail.If you match the two parts very carefully (no gaps) and you provide a large mating surface, avoid butt joints, and design the joint for mostly shear or compressive stress rather than peeling strees, soft soldered key mods/repairs can last decades. (I have plenty of evidence for this in my personal instruments). If you rely on a big glob of solder to provide strength, you'll buy yourself a lot of trouble.