Nature of the beast. Small engine+ big cam+ blocked heat crossover= poor/no idle when cold. Try raising the idle speed some. 900 rpm seems low to me for a 305 magnum in a 340. And unblock the crossover. You might be better off with a smaller cam anyway. I don't care for hyd cams that radical in such a small engine. You will be reaching the upper limits rpm wise of what a hyd is capable of. That cam profile is asking for 7000 rpm but the hyd lifters may not give it to you. My point is: not only would your idle improve with a smaller cam, but your car might be faster too.