I feel like a blithering idiot

Here's the scoop why I couldn't screw the sealed BB in:

There was interference from the screw that holds the cable guide on, that's underneath the BB shell.

Back when I had an open ball bearing set-up with a skinny little BB spindle having that screw stick down didn't matter. With a big fat UN55 sealed bearing unit, it did matter.

Backing the screw out: voila. BB screwed right on. That's about an hour of my life I'll never get back.

Chalk it up to a learning experience.

It's been said: You learn from your mistakes. But it's better if you learn from somebody else's mistakes.

I'll remove screw, grind it down, and reinsert.

Thanks for the help!