I am not certain but I do not think it is a direct bolt in.
It has been a while since I read up on this.
The 64 is a twin I beam. The 60 is a solid axle. A solid axle frame is narrower than a twin I beam frame. I think that the spring pads and shock mounts will be spread farther apart. Because the bodies of the '61 -'66 trucks were more or less the same, and I believe the non unibody trucks (not step side. Is that called fleet side?) of '61-'63 used the same bed that the '60 used, I think that the overall width should be fine. Don't take my word for it though. Seems logical.
Are you able to get any measurements?
Lawrenceville, GA