essentially the BB's are the same, i.e. The axle length and bearing spacing, it's the threaded cups that differ between the shell width BB's. You will get away with spacing a 68mm 2.5mm on the drive side, but, you may need to use a left hand cup from a 73mm BB or you may get away with it with the supplied item.
Typed on my phone, I couldn't check properly.
A 73mm BB will achieve the same chainline fitted in a 68mm shell BB with a 2.5mm spacer in the drive side.
A 68mm BB in a 73m shell will put the chainline (and left crank) to the right 2.5mm and you will probably need a left cup from a 73mm shell BB.
I have sqtp UN72 in 68 & 73, same spindle length, it's only the cups that are different, the BB body is the same for both.