Did you set up the cam gear and crank gear the way it was in the photos I scanned from my manual?
What Datsun manual are you using, maker and manual number please.
You actually don't even need a carb to fire an engine, one just pours a tiny amount of gas in the intake(teaspoon) and start it up, if it is back firing it's getting gas and will run if everything is set up correctly, I have started several engines over the years without the fuel pump, I fill the carb bowl full of gas, I turn the engine to TDC, I turn on the key, then I turn the distributor till the spark plug I got on number one wire sparks while the threads are grounded, I put the spark plug wire back onto number one plug, I then pour a small amount of gas down the carb, pull the choke on and I start it.
If anything is not right it will not start.
Does the engine turn over unevenly or very fast?