Cam choice

Hi, can I ask what year engine you have and are the pistons stock? I am new at this, but I thought all 340's excluding 1973 the last year, had at least 10 to 1 comp from the factory. I have a 71 that was positive decked at .026 and with stock x heads and I am expecting 10.5 to 1. Are the x chambers smaller than the j's Am i wrong? thanks walt