Sorry, but i can't find anything on XAG either. I doubt that its original to '63 though, because they didn't start using 3 digit codes until 1970. Try giving this code to your Chevy dealership instead of the casting number and see if you can get lucky this time.
Are the intake and exhaust manifolds cast as part of the head or are they separate castings?
Does is say "Mexico" anywhere on the block?
To the left of the casting number there should be another set of casting numbers, something like "CON2 F129", the F129 would mean June 12, 1969, for example.
