Parts express is awesome, If I can find the pick of my install with the relays on my car. The biggest reason for the relays I thought was the headlight switch, because halogen bulbs draw so much more power, it will overload the switch. One thing you can do first is get a headlight switch for either a 67-68 couger or a 69 mustang. These were designed for higher loads with the 4-headlight system. (solved a flickering problem in my car this way).
For the parts if you are ordering, I used the dual relay plug and 2 30A relays. I had to do a little mixing around of the wires in the plug, but it worked out fine. To make it easier, you might use just two plugs together instead of the single. Also, I'd recommend buying for future relay plans as the shipping will damn near exceed the cost of the pars if you are just getting these.
The Swede