For anything representing pre 1969 you want a cotton canvas M1 sling for the XM16E1. (remember the M1 nomenclature does not indicate M1 Garand, it just happens to be the nomenclature for that version sling)
The 1st contract for the seatbelt nylon sling was 1969 per Bill Ricca. It is also an "M1" sling, but made from nylon. Perfect for your later build.