apostate man,
The words in question found at 1 John 5:7 in the KJV are not found, to my knowledge, in any modern Bible translation (there may be one exception - a translation which simply removes some of the archaic words from the KJV). So the NWT is not unique in this respect. In fact, since this is the case, the WT is really attacking a straw man when it points out that these words should not be in the Bible - the Bible translators themselves know that! It has no bearing on the doctrine of the Trinity, however, as the early Church Fathers never appealed to this text in their dispute with Arius (the so-called Jehovah's Witness of the fourth century), yet they came up with enough Scriptural evidence to at least satisfy the Church that the Trinity is true.
For a standard Bible translation, I use the New King James Version. This is a translation in the King James tradition, but with the benefit of modern scholarship and modern English language. If you must use the King James, I do not recommend the Scofield Reference Bible. You can use a regular King James Version, or even one with marginal references, without relying on Scofield's notes. Scofield's notes are based on a system of Bible interpretation known as dispensationalism, the product of John Nelson Barby who lived in the early nineteenth century. It is the theory that God deals with mankind through seven distinct dispensations or ages. It is a theory that is imposed upon the Bible rather than one that is derived from the Bible itself.
As to how the extra words got into 1 John 5:7 in the KJV - they may have been a "gloss" - a notation made by a copyist to an early manuscript who thought he was clarifying the meaning of the text. Another copyist came along, thought the words were part of the text itself, and made sure they got into the next edition! The KJV, being based on late manuscripts, contained the gloss. But the earlier manuscripts which are used by Bible translators today do not have it.
Justin