6.5" speakers fit in the front. You'll need to grind and trim the plastic mounting rings. Easy job with Dremel or RotoZip. Only a tiny bit of material needs to be removed to get the needed clearance.

In the rear 5.25" speakers will fit but you will have to remove the metal bracket and enlarge the hole... yet another job for the Dremel/RotoZip. Definitly use heavy duty cutting disks... otherwise it'd take a while and you'd be inhaling more metallic dust than you cared to. Been there done that.

Randy, how soon do you think you'll have a proto of that sub?