Anton,

This may not help as it's not vintage-specific, however bending the spring (simply by pivoting the arm while mounted to the frame) is a common way many folks adjust even modern cantis and v-brakes. I would certainly never cop to this myself, but I hear it works without incident If the springs are at all similar I'd imagine you'd be fine.

Very curious to hear someone chime in who has experience with MAFAC brakes specifically.

Good luck, that 650b project of yours looks absolutely stunning by the way--hope to see it (and you) around town soon,

Jonathan