You know, I've thought along those lines but I've never been able to make a rigorous connection between the longheads and the Nephilim. Consider their typical characteristics:

Longheads: long skulls (obv) and tall but otherwise normal human bodies. Infrequently, the skulls have hair and it appears reddish. Apparently lived freely in community with humans.

Nephilim: giants, six fingers, six toes, double rows of teeth, red hair, often violent and cannibalistic, sometimes lived with humans but more often in separate and often hostile communities.

Not much overlap except for the red hair, which is very intriguing but far from conclusive. In years I've never stumbled across evidence as to what their real relationship might have been.

Also, there's the issue of beauty and it's standards. The Nephilim may have been big and powerful, but I don't think I've ever heard them described as beautiful.

Saul is described as "tov" and "bachur". Translations vary, but in any case I think he is being described as attractive. Well, today as in all times, beauty standards are set by the elites in society. Longheads burials have often indicated they were the elites of the day, so you see how it ties together that Saul had "movie star good looks".

In my view, we still have a mystery on our hands.