If this is an option, take careful measurements. Some of the early 64's did not cycle the length of the 64B's and thus the magazine/feed ramp assembly is too long, giving feed issues. I have this problem as we speak. Looks like it's a job for the mill, only to lengthen the distance for the bolt to travel (and make a new safety detent).If your mag is the older one made of plastic, you can convert your gun to the new type of metal mag.
Pm me with your email address and I will send pictures of the assembly.