The 63 and 64 from bumper end assemblies are similar in the way they bolt up and you can take the entire 6 pc 63 and replace with the6 pc 64 or vice-versa.
Here is the lowdown on the 63 & 64:
The 3 uppers are basically the same from 63 to 64.
The upper left and upper right is exactly the same. The upper center is the same except the tabs on the 63 upper center have holes for mounting the lower grill.
The 3 lowers are different from 63 to 64.
--> The lower ends (buckets) are wider/fatter on the 64 and where they mount up to the center lower is shapped different between 63 to 64.
--> the lower center on the 63 has a more pronounced/raised shape, whereas, the 64 lower center has a more contoured and smoother lower center.
What is a direct bolt on from 63 to 64?
--> Any of the 3 uppers are interchangeable as mentioned above.
--> You can take the entire 6 pc bumper end off one year and put on the other.
--> You can take the 3 lowers from off one yeare and put on the other.
I've heard of people forcing the 64 lower ends onto an 63 lower center and vice versa but you will need to do some bending to make it work.
As far as mounting a 62 or earlier bumper end on a 63 or 64 I have unfamiliar but would definitely think you would be more likely to do so if you replaced the entire bumper end. As mentioned earlier and above, even the 63 to 64 lower sections are different and are direct bolt on.