IMHO, the 'ring' headlights look pretty out-of-place on a mostly stock vintage Mustang. Some of the heavily modified and modernized ones look all right with 'em. But that's a matter of personal taste I guess! Keep in mind how pissed off you get when you are greeted with someone that has 'way too bright' headlights, even on dim. A lot of these put out a ton of watts and are not very friendly for highway driving.
If you get LED headlights, make sure they have a quality heatsink. The real problem most of these face is inadequate cooling, which can lead to a lack of brightness, or premature failure. They make them with a mostly 'stock' appearance as well as the 'ringed' versions.
If it's just 'smooth' glass, without the little Fresnel bumps and flutes, you should probably avoid it. Those are there to diffuse the light, and focus it on the road, away from other drivers' eyeballs. They work WAY better. (the bumps are on the inside)