Have you checked your picture post? (it's full of porn offers - I assume that's from the host you're using to post the pic) Anyway, when I stopped checking out the 'other' stuff, you can drill the 3/8" hole just about anywhere after the "tee" that the (I assume is for a slide line) - they only take up ~ 1/2".
Hayward says that the influent line should be before the filter... I think that's stupid - why run unfiltered water through the chlorinator???? - however, I've always just done as they say and put it on the pipe going from the pump into the filter. Because of the flex pipe, I'd drill into the coupling just above the pump outlet- the 'saddle clamps' that come with the unit are large enough to do it that way, so you have something solid to attach to.