As has been mentioned, I don't recall any mention of his religion. Could he be Catholic? Could he be Jewish? Could he be an atheist? I've never read anything that pinned him as a protestant or Anglo. (not saying there wasn't something over the 60 years, but I never noticed it).
But my point, more broadly, is that our society seems to want to put people in racial boxes (such as "WASP"), but genetics, history and the fact that people aren't thoroughbreds and they have the ability to cross racial lines when they reproduce makes those boxes meaningless.
Let's consider Oscar Isaac. If we just had a photo of him and nothing else, we would have no idea what his ethnic heritage was. He generally looks Mediterranean, but what's that mean? He could be Italian or Jewish or Spanish or Greek... at some point during his family history. But the Roman Empire spread up to Germany and over to England, so he could have those physical features while tracing his family back to Germany or England.
Or he could be Spanish, but the Spaniards spread out to the Americas, so he could trace his family to the Americas. Or he could be Jewish, but Jewish people have spread out from Israel over the past few thousand years.
Bottom line, Oscar Isaac looks the way Reed was drawn, so no matter what Reed's heritage or Isaac's heritage, he can play Reed very easily.
And he's an actor. If you really insist that Reed needs to be a "WASP", Oscar Isaac can play that.