Generally, Crutchfield does their homework. If they say a 5" will fit, the it's a 5" you need, or some sort of spacer ring to make a 5 1/4" work in the same hole.
I'm sure someone here has put larger speakers in their Gen I, and will post up about it.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes