If I can figure how to get mine apart, mine have the lower eye welded on and don't match up with the Betor or Sebac shocks found on other SB's (found this out after doing a search on this site for how to take mine apart). If I can get the shock apart enough to open up the lower tube of the shock, I may start experimenting with adding springs inside the tube to see if I can get the shocks to stiffen up. Depending on how that goes (or doesn't), my next step would be to cut out coils and re-stretch the spring. I would like to keep the OEM look as much as possible, yet stiffen up the shocks since I am a heavier rider.
My last resort will be to order a set of shocks from Treatland.tv. They're more expensive than the set that the surplus/Northern Tool sites carry, but they are the correct size (340mm eye to eye, roughly 13") .
I can't vouch for them that they are stiffer than OEM, but they are at least adjustable and
appear to have a thicker spring than the ones on my SB.
https://www.treatland.tv/xtreme-black-adjustable-shocks-p/xtreme-340mm-black-shocks.htm
These are the Northern shocks, 12" eye to eye is 1" too short, so some kind of adapting would need to be done.
Adjustable Shock Absorbers (sold as a pair) 13 1/4in. Length | Shocks| Northern Tool + Equipment
Paul