My friend is giving me a MSD 6a and i'm wondering if i get the harness from MSD if I could use his MSD box on my car?

 

Yes, they are not application specific. (I am 99% sure )

 

On the side of the box there is a little black cap with a screw in it. If you open that up, there are two little wires. The box is set up from the factory for a v8. In order to it work with 6 and 4 cylinder engines, you neet to cut one or both wires respectivley. To restore it to v8 operation you will need to send it back to MSD to reconnect the wires, or open it up yourself. I would send it back to MSD if it were me.

So the answer is yes with the above exception.