I have a '91 Trek 950 that I am rebuilding into a single speed. I have replaced everything in the drive train except the bottom bracket.

When I spin the cranks I can feel some grinding and certain spots are harder to turn than others. In addition, when I put hard pressure on the cranks, either side, there is a loud popping noise and the cranks seem to jump a bit. All that to say, I think I need a new BB.

I just purchased a set of square taper Deore 175mm cranks. I know I need a 68mm BB, but I don't know the spindle length I need.

This is for a singlespeed and I am using the gusset singlespeed conversion kit with the 14mm on the outside and the 19mm spacer on the inside....
I know I need the UN-73 but 68x?? I think my current spindle length is 122.5, and I think I need 110- or 113, if I'm reading sheldombrown.com correctly. the cranks as they are on my old BB seem kinda wide, but the chainline is good right now, with a 2mm spacer in between the spider and the chainring.

Any info is greatly appreciated...

How do I know for sure?