I'm trying to replace the completely mauled cup & cone bottom bracket on my Benotto 800 with a sealed cartridge bb. The axle says 3P, so I used Sheldon Brown's size chart to figure out that I likely needed a 68x122 bb.

I got a Shimano 68x122, but it slipped straight through (didn't catch on anything). I'm somewhat perplexed now. It's a 58cm frame, so could it be that I need a 73? It seemed to slip through too easily to be Italian (70)...am I just completely wrong? 122 seemed to be correct from eyeballing it (am I wrong there, too?).

Wasn't sure if this should go here on in the Mechanics forum.