Italian bottom brackets and modern cranksets - a compatibility guide?
Spindle length and chainline are immaterial to it being Italian. The center of the spindle is at the center of the shell either way. If the chainline is good now and you're using all the same parts, get another JIS BB of the same spindle length in Italian.
If you wanted to tweak the chainline, it's the same process as any square taper crank. Use your existing crank and BB to figure out what length spindle puts the chainline where, figure out the chainline you want, and change the spindle length by twice the difference.