I would imagine that the chassis will have "knock outs", a hole almost, just not fully made it is in the form of a disc of metal, that is only held on by a few little tags of metal where it hasn't been cut all the way around.
You should get fitting diagrams with the bar you buy though, you always do with witter as I use them.
Only advice I can give is put it all on, then take off and zinc prime everything where metal is exposed, then add sealant then bolt back up into place, electrics are easy on your age car, std socket is a 12N, supplmentary socket for fridge/power etc for caravans is 12S, if you need a wiring diagram pm me your email.
tabetha