To be honest your post is a little confusing. Just to set the record straight, round headlights came on early 78' Customs & Rangers only and squares came on 78' XLT's. About halfway thru the year they switched to squares on all of them and ALL 79's came with squares. so really only the early build 78 Customs & Rangers had rounds, everything else came with squares. You can retro ANY 73-79 F-truck grill SHELL (meaning grill shell, grill & lights) onto any 78/79 Bronco. Yup, I've seen the whole 74' shell on a 79' Bronco gig and to me it just doesn't look right, the split-grill 73' look just seems absurd on a 79' Bronco but nobody appointed me as Ford's fashion cop. Now if what you're saying is that you want to add square headlights to an early 78' Bronco by swapping to a square headlight 78/79 F-truck grill shell then sure, you can do it as a bolt on.
To repeat for the 2nd poster, YES, *any* 73-79 F-truck grill shell, grill & lights will bolt onto a 78/79 Bronco. Just how that grill looks on your Bronco is a whole different matter, but sure, they'll ALL bolt on with no modifications.