if i wire an amp that puts out 300 watts to two 120 watt speakers what is the likelyhood of blowing the speakers

well the amp rating most of the time is PEAK power, not RMS. you can adjust the gain on the amplifier to tune it down. Or you can adjust the Fade on the speakers on your head-unit

amps usually don't put out that much power unless they are wired properly gettin all 14.3v. So i would say not likeyly, unless you hook the amp up wrong to your speakers