Better to use a template.
You can create an email template (blank) with fixed width. Then, everytime you create a new email, you can use that template.
I have many templates that I use according to the need but yes, I use fixed template so that, text/images stay fixed.
This should help: http://community.marketo.com/MarketoArticle?id=kA050000000KyqNCAS
-S
Not sure if you have found the solution! Yes, using fixed width in the template and the outter table to be fixed should solve the problem.
Email looks different on different email-clients but this table based lock system should work.
Thanks,
Syed