The ideal place is at the lower radiator hose. This isn't always the easiest place, though. I have mine mounted in the t-stat housing on both my '66 and my '69. You don't want it in the intake. You want it after the t-stat.
You DEFINITELY want to use a relay. Any fan that can pull enough air to be efficient, will pull a lot of amps. You'll also want to upgrade your alternator.
An override switch is nice, but certainly not necessary. I don't have one on either of my cars. It would be nice to have at the track, though, to run the fan sitting in the staging lanes, then shut off when I go to the line.
VMF Jedi Knight who's spirit will be with us forever.