I just copied the 3-line block into my post, hightlighted it, and pressed 'Monopspace'. If the bars line up on preview it should line up when posted.

You said it didn't line up on preview before, and it appears to me that line 2 had single spaces. Why that was I can't guess.

Copying and Pasting text doesn't work the same in every operating system and every program that you copy from or to. Sometimes you end up copying formatting as well. In particular there are ordinary (ascii) spaces, and non-breaking (unicode) spaces, as well as html non-breaking spaces:   Only ordinary spaces get changed to html non-breaking spaces by the MBT Monospace button. If you copy and paste from mudcat itself you might get unicode non-breaking spaces.

None of that should matter. If the preview looks right it should post correctly. If necessary add or remove non-breaking spaces.