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FoMoCo went to square headlights in '78.5. I have a 75 grill on my 79, as far as I know you just have to drill some new holes for the headlight buckets. I don't know if you can just slap on a circular bucket over a square setup.
If my memory serves me correctly - I'm comparing a 79 to a 78.
The headlight brackets should bolt up correctly and the trim pieces should screw into those brackets.
Sorry about that Dennis, I must have been tired, I have always preferred the rectangular headlamps to the round headlamps, however my grill was all busted up and dented in the aluminum, the 75 one was free and nearly perfect shape.
I think if you get a set of headlight buckets and mounting brackets from a 78 with the round headlights, it is a direct bolt in in place of the rectangular brackets. 77 and older have a different bracket because the headlights are mounted lower.