lowridin63, the shop manual for a 1964 truck starts with the 1963 manual. The 64 manual is only a supplement. According to the index for the 64 manual section 2 "Frame" indicates that there is no change between the two.
Also when I mounted my cab back on my 64 I used the dimensions from the factory assembly manual (located about 1/2 way through it) to verify my frame was good to go. I judge that this particular section of the assembly manual is for 1962. These dimensions matched what my 64 frame had.
I believe that all the 64 front end sheet metal (fenders, hood, etc.) will fit on a 63 as well.
So it seems to me that the biggest concern you have is are you ok with the look of a 64 cab on your 63.
That and you will have to always remember what year parts you will need depending on what part of the truck you are working on.