First you need to decide what you want out of your car. What do you want to do with your car street car, highway cruiser, drag race? Have you driven or rode in a 340 car? Is that what you want or are you willing to put up with a lot of extra stuff dealing with a race motor? Unmilled J heads are around 73cc and unmilled 68-72 heads are about 68cc. What gas do you want to run regular or pump premium? Now you can decide on a compression ratio and buy pistons accordingly. 340 cams are in the 212-222 range of duration and .450 lift. That will put you in the 14-13 second range in the quarter mile.