74 Dart won't start - ignition switch - maybe starter relay?

In run position, do you have +12v to the coil?
This is what you need to investigate first. If you have the key in the ON position, and while bypassing the starter relay to crank the engine, you get no spark, you need to find out of you're getting power to the ballast and ignition coil. Sounds like you may have created a bad connection somewhere in your engine installing procedure.