Hard to hide them without a battery if that was your intent. On a 68 Dart, I would mount them on the back of rad support, pass side. Shortest runs and can be put down low above frame rail to keep out of sight.
Street cars I like the relays in the engine compartment to keep them out of the heavy elements (rain). If you never drive in the rain, then on the front of rad support works as well. Pick a spot and I can build it for you!