If the speakers have a thick front gasket, and there's enough room under the rear deck, then you can just mount the speakers to the bottom of the deck. The tweeter pod should be in the middle of the 6x8" opening, and the fact that the opening is smaller than the woofer cone won't really hurt the woofer output.
The speakers need a thick front gasket so that the woofer isn't pressed up against the rear deck metal when it's mounted. Also, it only works if the rear deck is flat enough that the speaker gasket presses up against the metal without leaving a gap.
I'd advise using some insulating material, like adhesive weatherstrip, between the top of the speaker and the rear deck metal. I'd also advise using a damping material like Dynamat on the rear deck, to reduce vibration and seal any holes in the deck.
bigger isn't always better, so I'm not sure why you would have bought speakers that don't fit intentionally...I would just get the right size speakers. Or get a set of 6-1/2" speakers and an adapter plate.