Why did some 79 trucks get four headlights and some only get two?

Will the 73-76 truck grills fit the 79 two headlight radiator support?

56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bees, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d