...like Mr IGH, I had some seating issues in the frame I used, so if you have the luxury of time, you might want to install the conversion kit first, and then measure the resulting width of what you end up with in terms of whether a 68 will work, or whether something a little wider or more universal (68-73) might be advisable in your selection of a spindle unit. Also, try to be smarter than me and put the correctly threaded sides in their respective positions the first time.
Otherwise, as stated above, it's not a big deal what you use, so long as you get the correct spindle length for your chosen crank that maintains a good chainline.
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