The only ways I know of to fit a speaker into a hole smaller than itself is to either grind out the hole (see rayray's DIY) or buy an adaptor bracket. Try http://www.sounddomain.com, they've got several adaptors for speakers, but I can't remember if they go from large to small or not.
The MA70's after '89 had the option of the 6spkr system, and in 1992 had the option of the "System 10" (10 speakers, Infinity setup) however, it's incredibly rare.
As for fitting speakers in your MA70, it depends on the year it was. The pre-1989's came with smaller speakers up front and in the rear. post-1989's had 6.5"'s in the doors, however they weren't enclosed so some argue that sound quality was much poorer. You can swap the pods from a later model into a pre-1989 with minimal effort.
As for the speakers in the rear deck, they're difficult to get at and while I've seen 6x9's stuffed into the rear, it was very tight, and it was quite a hack-job.