79 truck questions

I'm looking at buying a 79 stepside. It has a single round headlight on both sides. I thought 79 was the year they switched to dual square headlights on both sides. The center of the grille looks exactly like the 79 grilles with dual headlights. Is this model specific, or a half year kinda thing? The truck I am looking at is a 318 Custom 100. I am glad in a way because I would like to put a 78 grille in it. It should be an easy swap because of the single headlights, right? I will probably turn it into a Midnight Express clone, but keep the 318 2bbl. My Duster should keep me in good graces with the tire manufacturers, oil companies, you know the deal. Thanks in advance, Keith.