It's important to keep in mind that it's not just the Alexandrian manuscripts that lack the Comma Johanneum (which is the phrase used for the longer reading of 1 John 5:7-8 found in the KJV and NKJV), but also the vast majority of Greek manuscripts don't contain it either, as well as ancient translations into other languages, like Coptic, Syriac, Old Latin, Armenian, Georgian, etc, also lack it. It only appears in several very late Greek manuscripts (in the main text itself and not as a later marginal addition) and the first two editions of Erasmus' Textus Receptus didn't even contain it. The only reason he included it is because he was pressured by the Catholic Church to include it, as it appears in late Latin Vulgate manuscripts. I wish I could agree with you that it was originally part of 1 John, but the manuscript evidence is strongly against its inclusion. Even some of the English translations that predate the KJV, like Tyndale's 1534 , the Coverdale Bible 1535, Matthew's Bible 1537, Great Bible 1539 and Taverner's Bible 1539 all have it in brackets, as they too questioned its authenticity. Interestingly, in the Taverner's Bible, it has a marginal note about it saying: "This that is printed in other characters after the judgment of Erasmus in his annotations be not the words of John, the writer of this Epistle, but came to be put in, of some other."