MUST BE PAIR
I can confirm that your logic board and communications board MUST be pair to each other, otherwise there is NO way for you to activate the iPad.
This was my experience:
I bought very cheap and iPad 1st gen (64 GB, 3G) from a friend because it was totally death (no screen, no charge, nothing) and he didn't want it anymore. I did a diagnosis and the logic board was the culprit, so I bought another one from PowerbookMedic (used, in grade A condition).
I was happy to see that after the logic board replacemente the device came back to life, it charges, screen and digitizer works, etc, but it did not recognize my SIM card. I got stuck at the activation screen (after a lot of things and frustration) with the dialog: "There is a problem with your ipad"… indicating that I should take it to a Authorized Service Center or so.
So, as an Independent Service Provider in NIcaragua I call the AASC here and explained to manager my situation. After 24 hours he call me back, confirming that the boards (logic and communications) MUST be matched in the Apple Activation database, and that's why you can't activate it… moreover, she explain me that there is nothing they can do to change this in the database, because there is an administrative rule (not a technical one) that says that can't do that either… therefore that is something they can't request from Apple to do so.
By the way, the "Apple Technician Guide" sucks, it did not even mention the possibility to open the device. So no hope there. I guess that Apple want us to see this devices as dispensables.
So, basically I'm stuck there, and will as PowerbookMedic for a refund, because this is something that part retailers should know already (including iFixit) before selling you this without the match part.