Examples of the "Gutenmark" approach abound at http://www.freekindlebooks.org
-- which also has the start of a "mirror" of project gutenberg in MOBI format at http://www.freekindlebooks.org/MobiM...obimirror.html
The approach I would recommend for the "casual" user -- ie the personal reader of Project Gutenberg texts rather than someone who wants to republish those PG texts -- is to get the EPUB version of the book from PG and then run the Calibre "any2mobi" command line tool on that file. It takes me literally about two minutes to pick out a book from PG and do this conversion.
For Kindle I wrote a little command line batch file to call any2mobi with Kindle-centric parameters:
"c:\program files\calibre\any2mobi.exe" --no-justification --cover ..\pg.jpg --dest-profile Kindle %1
where "pg.jpg" is a little dummy cover I insert because I do not like how any2mobi tries to gin up a dummy cover.