Help with modifying MOBI file

Ladies and Gentlemen,

I left my warm lair at Kindle subforum and come here to ask for help.
I need to achieve some specific MOBI modifications and my level of Python-Fu prevents me to write it myself.

I have 500mb MOBI files created by KindleGen (Hybrid M7/KF8 headers) and I need:

1. Remove Source header. To decrease file size ~50%.
2. Change header field 501 to EBOK. To remove that ugly Personal tag on Kindle Paperwhite.

Currently I achieve both goals with modified KindleUnpack. It works correctly - KindleUnpack is awesome but it is way to slow with so big files. KindleStrip remove Source header much faster... but then I need to modify headers and I can't. All metadata editors I found fail due the fact they edit only first (M7) headers. KF8 headers are untouched.

I would be very gratefully for some Python snippets to edit that field 501 in both headers. Something slimer than parsing entire file like KindleUnpack. I need a scalpel and KindleUnpack is hammer :-) Patch to KindleStrip that set field 501 in both headers would be perfect.

Free candy to anybody who can help me.