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Round headlights came in 1978 trucks, square headlights came in 1979 trucks.
In '78 only custom level trucks had round headlights. trucks ordered w/ranger trim received rectangular headlights. in '79 all trucks had rectangular headlights.
If your truck had the option for round go for it. This kind of reminds me of when I was in the Volkswagen scene. There were always a couple of cars out that would be like a 73 super beetle with the tiny 61 and earlier "snowflake" tail lights and earlier lowback bucket seats. Yeah they fit but it just didn't look right. lol This isn't anywhere near that drastic, i'm just a big fan of correct for the year type of stuff. But then again it's your truck
As far as illumination of round vs. rectangle, I have heard that the round headlamps will project the light better than rectangle. It has to do with the natural reflectance of the light off the inside of the headlamp. there must be some trueth to this as if you will notice alot of the newer late model vehicles use more of a round headlamp design (even though they are installed in an aerodynamic plastic cover) than rectangle.
I've started to turn my crew into a Harley-Davidson model truck only with era correct emblems. For a cool twist I put 01 and up H-D 7" round headlights on the truck. They fit and I can replace the bulb with H4 super white. And they have a little bar and shield in the center of the bulb.
MY truck is a 78 and has the round lights. I like both the round and rectangle. I think the round is a little more distinct than the rectangle. Atleast here where i live, cause everyone else prefer the rectangle. I would keep it round if thats the way it came. Just my opinion though, either way, it will look good!
Rectangle is correct for the '79. They are readily available and cheap to replace when needed. Both NAPA and ACE carry the H6054 model dual beam sealed rectangles here in Wyoming and I pay $12 for good NAPA or GE models.
If you are going for accuracy, rectangle on the '79, if you are going for aesthetics, go for whatever you like best!