The rulebook does state it, and the answer is you can ALL shoot at the target with all ranks. The rule is actually on page 9 under "Hills and elevated positions" and confusingly needs 2 sentences read together,
and they can always shoot (and be shot at) over models that are not on the hill, in the same way as described for large targets This means thar models in the rear ranks of units on the flat may shoot against...
The problem is they never actually state "large target" anywhere else, apart from the to hit modifiers section.
So correct answer: all ranks can fire at a large target