Mine ['68 model] is also using the front holes, however a '69 chassis I have seems to have used the middle holes.
Impossible to use the wrong holes as the inner guard holes won't match to the rad support as well as lower valance bracket.
On the assembly line, the entire front clip was pre-assembled and moved onto the chassis/body as a complete assembly. The radiator support would then automatically line up with the correct holes.
69'ers, can you confirm, middle holes or front holes?