When I put seat belts in my Bonneville, I did some research and found a factory memo sent out to the dealers that described how to install seat belts. It turned out the factory put small dimples in the floor pan to show where the holes for the seat belt mounts were supposed to go. The dimples were pretty small, like what you would make with a center punch, but when I knew what I was looking for they weren't too hard to find - I cleaned off the floor pan and looked behind the seats in the general areas where I thought the mounting points would be, and there they were. My car (a '61) had provisions for both front and rear belts, so I'd be pretty confident your '64 has them as well.
I used repro lap belts from Ssnake-Oyl, which were very good quality but not particularly cheap (then again, what price do you put on safety.) They came with hardened bolts and large round reinforcement plates (like big washers); I simply drilled the holes through the floor pan and bolted them in place, no welding.
http://www.ssnake-oyl.com/