I have a generic POH for a PA28-161 Warrior II, and there is naff all in there about the radio fit. The closest to any kind of info is a line drawing of the panel with the radio stack shown (as the old clockwork type radio's).
As has been said in other posts, radio and nav fit is very much specific to the individual aircraft and can vary widely within the same club fleet. I fly Archers and Warriors - the newest run dual GNS430's whilst one of the oldest Warrior II's has a standard 'digital' Bendix/King radio as box 1 and a clockwork radio as box 2.
If you are unsure, when with your instructor, ask for a few minutes familiarisation with the radio you will be flying with - soon you will realise that whilst they all look different, the operation is pretty much standard and you will then be comfortable with whatever box you have to use. Caveat on the integrated Nav/Com/GPS boxes like the GNS range as it can all go horribly wrong by just stabbing at any old button
