Stick em behind the battery tray if you have space, easy place to choose cos youre already to tap straight into the two power feeds and the battery source.
Those Bosch relays are perfect, don't worry about the where the feeds are coming from, just use your test meter to isolate the dip and main beam feeds and attatch as the diagram.
On a 72 there is a different slam panel of course, so you'll have to experiment where to site them.
Obviously a water resitant cover or something similar would help no end, but I've seen a lot of em just screwed to the rad support panel at the top right where the lighting harness goes thru