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There have been several discussions on this in the past...
The 1978 dealer brochure that I have says that rectangular headlights came on the XLT models, while the Custom models had round lights. In 1979, all models had the rectangular lights.
My 1978 F250 XLT Ranger 4x4 has the rectangular lights.
Actually,In 1978 the rectangular lights became standard equipment on Ranger & above trim line's.
http://www.clubfte.com/users/mil1ion/78headlights.JPG
The round lights were on the Custom's.