Timing problem

[sgt1022]

Ok I had a previous thread for ignition problem got that sorted out. I cant seem to get the timing down right on my 454. This is my first big block chevy. I have done everything I know to do. I have #1 on TDC(compression stroke), Timing mark on cam is at 6 crank is at 12. Distributor is pointed toward #1. Installed wires in correct sequence it is backfiring through carb. So that tells me that an exhaust valve is open when firing. Camshaft is a Summit 1301, advertised lift is .502 on the intake and .501 on the exhaust. advertised duration 277 intake 287 exhaust.