Can I substitute a square taper BB with different shell width and spindle length? I'm swapping out a worn BB and crankset from a late 90s hardtail. I already have a new triple crankset that specs 122.5mm spindle length for proper chainline. However, the frame has a 73mm BB shell width, and these days a 73 x 122.5mm square taper BB is unavailable (sold out). To maintain chainline, I'm going with a 68 x 120mm square taper BB instead. This will result in the non-drive-side being 2.5mm closer inboard, but I think I can live with that. Is there any drawback to this plan that I might be missing?

The solution you are proposing isn't ideal but should work.

However, depending on the bottom bracket unit, the non drive side "lockring" might have a shoulder on it, preventing it from contacting the bottom bracket main part fully and therefore not supporting it as well as intended. Many types don't have a shoulder and this is a non-issue in that case, it will just disappear into the frame a bit (the threading generally allows this).

Otherwise you should be golden.