Singular or plural verb?

Hi
I know that in sentences like "What/All I want is/are two girls, two Ferrari and two million dollars" both is and are are used (but I perfer are). Now, what happens if I have a subject that is more specific than all or what? Examples:

• The best thing was/were those belly dancers. I really liked them.
• The problem was/were the last three lines in the source code. I changed them.
• The source of my confusion is/are these sentences. What verb sounds natural?

So, plural or singular verb? I think that plural verbs sound better, but I'd like to hear some opinions from native speakers. What is used and sounds good in American English?

Thank you.

Thanks.
Anyway, I think both a singular and a plural verb can be used in cases like those. In fact, I've seen both in similar sentences, it seems without any apparent reason. Here's what could be an explanation, let me know what you guys think:

When we use the verb to be, there's not a real subject. Example: This is my car ---> My car is this. Here, is is a kind of equal sign (=), like in mathematics, so it's not clear which one we should consider as the real subject.
So, in sentences like The problem is/are these lines of source code, people choose a verb more or less at random, depending on what they are thinking of, and they could think of either the lines or the general concept of problem (and the choice is usually unconscious).

Waiting for your opinion, thank you in advance.