I basically just print the email using CMD-P this will display the dialog. On the bottom there is a button with PDF, where I changed the default action from 'Save as PDF' to Upload PDF to Notes.
I also have a shortcut that will parse the "Date: ….." line from the PDF and puts that date on the Clipboard, so I can also provide the date easier to read in the note itself.
The great advantage of storing important emails in Notes / FN is that I can tag them.