obd2 magnum swap?

BCM isn't an ODB2 thing per se, but the security system on the truck relies on the VIN and security codes being the same to allow the engine to run. That's if the truck has an immobilizer security system. Some have it, some don't, but by 2001 I think it almost assuredly has it. You can get the PCM ( powertrain control module ) programmed to remove the security feature, but you'll need to send it somewhere to have it done. There is a company in Lantana FL that does it. I had to have my PCM done, took them about a week including shipping time.
.... I see aaadave beat me to it about the security thing ;)

Oh, and the ODBII port wires come from the PCM not the BCM so I'm almost sure it will work without the BCM although you might get some codes for not being able to communicate with it if you hook a code scanner up to it.

x2 on the factory service manual ( download if you can find it online, x2 on truck forums )
As for what wires you'll need... that depends on how much you are integrating.
You stated that you have the engine harness from the truck, but there isn't really an "engine only harness". If you have all 3 plugs that connect to the PCM and the PDC itself ( Power Distribution Center / Block of fuses/relays that goes under the hood ) then you should have all of the connectors that control the engine and trans.

I was going to go into some over simplified "do this, that, and that and you're good to go" thing, but I don't want people to read and rush into it without understanding what all the wires do.
Reply to my PM and I'll send over some color diagrams of the "simple version" if you want.

Oh, and welcome aboard!!! to another local