About the only way of achieving what you describe would be to include the letters in the Headings themselves, with a tab between them and the rest of each heading. For example:
My Project Heading → XYZ
where the → is a tab character. You might employ aright-aligned tab-stop in the heading and/or TOC to achieve the desired alignment.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]