68 is the width in millimeters of your bottom bracket shell. Any 68mm bottom bracket will thread into your frame properly. The spindle length will determine your chainline. For a single speed, you want as perfect of a chainline as possible, meaning that the chainring is perfectly in line with the rear cog. Assuming the crank you are using has the same chainline as the crank of the standard RR, a 109.5mm BB should fit just fine. The effect on the chainline should be only half the difference in BB spindle length, or 0.25mm, a negligible difference.