I would say a bone yard and find the brackets off a 390. If you google "352 ford alternator swap" there are some helpful links but basically all FE engines should be pretty similar and finding the brackets at the bone yard would probably be the easiest and quickest way to get it done.
FYI, there are also kits available on evilbay if you look there
I think there are some bolt holes that are not in the block or head on the earlier FEs that are on the 65 and up for a alternator so you might have to do some modifications.
As a last resort, a powergen from power master. Looks and mounts like a gen, but is an alternator. Bit pricey, but if they make them for y blocks, they probably have one for FE's. Hopefully, they have a wiring diagram for bypassing the regulator.
yes the 65 and up had some bolt holes the earlier ones didn't.if you could find a 64 with alt,it bolts on the same place as your gen,but only some late ones have it.