Since no one else has chimed in here I will give you my two cents. If I knew what I was in for before I decided on a 460 and C6 to replace my 289 and C4 in my 64 Galaxie I would have gone for a FE series big block. However my attitude may change after I get this thing running, If you decide to go 460, expect to run into more headaches than you can imagine and more work to go along with the headaches and sore back and hands. If you are young and ambicious and have lots of money and time maybe the 460 would be fine. There is plenty of help and information on this wonderful forum if you go either way. Just my 2 cents.
I agree with boxtop. You just gotta stay focused on the goal Bill. I can absolutly assure you, the first time you nail the throttle in your 64, your gonna be so glad you settled for nothing less than the 460.
Usually, its easier to replace the same engine you already have. In this case, it's the 390. It's a good engine, and lighter than the 460. However, if you like torque, a 460 is a good choice. Depends what your goals are. I really have not heard any good things about the 400 engine in terms of performance.