You got two "timings" to be concerned with. One is spark timing, the other is valve timing. Either will cause backfire thru carb if one is off.

Establish TDC #1 on compression stroke at the crank pulley and see if distrib rotor is pointing at #1 plug wire on the cap....if it is, then pull the valve cover and see if both #1 intake and exhaust valve rockers are loose like they should be and the valves are fully shut with the cam on it's base circle.

If all looks ok, then delve deeper into cam timing before you slap the valve cover back on. Make certain you installed the cam sprocket and chain as per specs.

OOps, also take time and make certain plug wires are installed correctly. You could be dead-nuts on spark time and valve time, but if plug wires are crossed, it will back fire thru carb.

She should never backfire thru carb. Not even when distrib is installed properly, but at extreme limits of travel allowed by distrib lock bolt.