What I was trying to illustrate by comparing that verse between those two different bible versions is that:
One is saying 'I am crucified with Christ' (KJV) and the other says 'I have been crucified with Christ' (NRSV)
There is a difference here in the wording. 'Am' crucified conveys a permanency that 'have been' does not. Once saved always saved. If one has to conduct works in order to maintain his salvation then why did Jesus die on the cross?
I think you're actually being misled by the archaic English. In early modern English "I am" can mean the same thing as "I have", as in "I am arrived". (It's easier to see this in French, where a similar construction is still used.) In any case, the NRSV is absolutely correct here. The Greek word is συνεσταύρωμαι and that simply does mean "I have been crucified". It's in the past tense. I think that the King James translators were also trying to express this but the passage of time means that modern readers misinterpret them.
Now you may prefer "I am crucified" in the modern sense of that phrase, but that simply isn't what Paul wrote.
The next difference I was trying to show is in the following:
The KJV says 'I live by the faith of the son of God' whereas the NRSV says 'I live by the faith in the son of God'.
Again the difference at first glance might be insignificant but they are both two different concepts. To live by faith of God means that that faith is given to you by almighty God whereas to live by faith in God puts the emphasis on man and disrespects God.
This one's harder as the Greek is really all over the place. Paul writes ἐν πίστει ζῶ, which is simply "in faith I live", followed by τῇ τοῦ Υἱοῦ τοῦ Θεοῦ, "that from the Son of God". So neither "of the Son of God" nor "in the Son of God" actually appears in this verse. Though obviously "in Christ" is a standard Pauline phrase, so perhaps the NRSV translators used it for that reason?
It's not a matter of who got the translation correct word for word but who got the translation correct meaning for meaning. Remember something said in one language may have no equivalent in another language or might mean something else entirely. Translators must take that into account in addition to getting the words correctly translated.
It's true that translation is never exact and that all translations are, to some degree, paraphrases. However, that doesn't mean that you should just go with the translation you like the most. It can still be the case that one translation is more accurate than another. And the way to work out which one it is is to compare them to the original language, where possible.