I put 390 emblems on my 352 Galaxie a long time ago. I eye-balled them to be just about center in the fender section that is below the lower crease, between the fender and door opening, and above the rocker panel.
If you have a 64 sales catalog, take a bird's eye view and see if center is about right.
I can take a copy of my sales catalog showing the emblems and fax it to you, if you wish.