Use 4 ohm speakers or you may be disappointed in the volume produced.
Quoted from nosro:
BTW, I don't know if I mentioned, but the Boss Audio 4x10 I ordered does NOT flush mount.
If the speaker is not a flush mount, then you can use a 4x10 speaker depth ring; -or pull the speaker panel out and using a wood router, rout out the panel "fingers" until the tweeter allows a flush fit. .