I would put it outside the quotes because the exclamation is yours; you're not quoting it. Same logic as this: Are they your "good friends"?

That is a good question. I would tend to agree with Steve above. It depends on whether you want to communicate that the exclamation is yours. Generally, it should go outside the marks. But, in that context there are two possible meanings 1. You are adding emphasis about being good friends. 2. The person you are quoting is adding emphasis about being good friends.

I don't understand your question well. Are these two different sentences? If these are two different sentences, I'll go ahead and explain the first one only because the second one is not gramatically correct (the noun "friend" is singular but the verb and the subject are in the plural) in any case, my answer applies to both sentences. You could enclose "good friends" in quotes or not and both would be gramatically correct. If you enclose it in quotes it would mean that somebody else refered to those people as your "good friends" and you are sarcastically quoting what they said; if you don't enclose "good friends" in quotes then that just means you are not quoting anything.