The front door speakers can be retro-fitted fine, with spacers. however, it's the rear speakers that will be tight. I suggest making a ported box for them and cutting out the stock mounts. that's what my plan is, better acoustic positioning, louder and more clear, and much cleaner. plus you have way more room behind the seats.
2002 Red Mazda B2300
Short Cab, Short Bed, Pioneer sound system:
DEH-P5100UB deck, dual USB/aux inputs, single CD, organic display, remote
4 TS-A6872R 40 watt nominal speakers
2 TS-SW2501S4 250 watt nominal 10" subwoofers