I need expert advice on an audio sytem for my silverado?

hi. i have a 88 silverado and im putting a system in it. i am building my own box and i was thinking about 2 12 inch kick l5 subs. do you think this is a good idea or should i just get 10 inchers. also. if i get the 12 inch kicker l5's i wanted to get a 2 channel mono amp to power them and i was wondering what would be a good amp to get the 600 wat rms to both of the subs. thanks and all help would be appreciated because i dont know much about this sub wattage and amperage stuff. the box i am building is going to behind my seat because its a regular cab. i already know the dimensions to make it so any help about the amp would be great. thanks