The width for externally bearing threaded BB's is not very important. What is important is the threading. If you have an italian BB it has the same right handed threading on both sides. You can in effect make an italian BB with external bearing BB's by buying two english BB's and only using only the right hand threaded external bearings.
In this case the difference between 68 and 70 is the difference in the two threadings. I have the italian version of this BB on my italian threaded bike and there is enough slop in the plastic housing in between the bearings that I suspect it is exactly same part. My guess is that the only difference btw 68 and 70 is the 68 comes with BSA threads( one right, one left) and the 70 comes with two right handed.
Since the crank you would use with this BB is almost certainly designed for a 68mm BSA BB, you will have to tweak the spacers a bit to get the optimal chainline.
Having said all that, unless you are working on a vintage steel bike made by an italian company, it's somewhat unlikely that you have an italian threaded BB.