I have a 350 in my El Camino with a mild street.

I have a 350 in my El Camino with a mild street cam. I put a Eledebrock 600 carb on and am getting back firing throught he carb also when I start it without the air cleaner on it starts everytime with one crank with it on it takes many cranks. Carb, dist, plug wires, plugs and air cleaner are new. Can't tell how to set the timing because of the cam.