We're still not talking about the same thing. I'm talking about the project you're using and need source for. You're talking about using other projects in another.
We're not? It sounds like we are... If the project that you're using and need source for is GPLv2, and the code you want to use is GPLv3, then you cannot use it (and it might not be good idea to even look at it -- depends on how litigious that other project is)